MFC r258046, r258047, r258050, r259035, r259036, r259037, r261842, r261843,
    r261844, r261845, r261846, r262194, r262522, r262559

r258046:
Fix a typo on a comment in ofw_bus_if.m, the default method will return -1
when a node doesn't exist.

r258047:
Move the KASSERT() check to the point before the increase of number of pins.

r258050:
Fix gpiobus to return BUS_PROBE_GENERIC insted of BUS_PROBE_SPECIFIC (0) so
it can be overriden by its OFW/FDT version.

Give a chance for GPIO devices that implement the device_identify method to
attach.

r259035:
Remove unnecessary includes and an unused softc variable.  While here apply
two minor style(9) fixes.

r259036:
Move the GPIOBUS_SET_PINFLAGS(..., ..., pin, GPIO_PIN_OUTPUT) to led(4)
control callback function.  This makes gpioled(4) works even if the pin
is accidentally set to an input.

r259037:
Fix the pin value reading on AM335x.  Because of the inverted logic it was
always returning '0' for all the reads, even for the outputs.  It is now
known to work with gpioiic(4) and gpioled(4).

r261842:
Add an OFW GPIO compatible bus.  This allows the use of the DTS files to
describe GPIO bindings in the system.

Move the GPIOBUS lock macros to gpiobusvar.h as they are now shared between
the OFW and the non OFW versions of GPIO bus.

Export gpiobus_print_pins() so it can also be used on the OFW GPIO bus.

r261843:
Add OFW support to the in tree gpio compatible devices: gpioiic(4) and
gpioled(4).

Tested on RPi and BBB (using the hardware I2C controller and gpioiic(4) for
the I2C tests).  It was also verified for regressions on RSPRO (MIPS/ar71xx)
used as reference for a non OFW-based system.

Update the gpioled(4) and gpioiic(4) man pages with some details and
examples about the FDT/OFW support.

Some compatibility details pointed out by imp@ will follow in subsequent
commits.

r261844:
Allow the use of OFW I2C bus together with iicbb(4) on OFW-based systems.

This change makes ofw_iicbus attach to iicbb(4) controllers in addition to
the already supported i2c host bridges (iichb).

On iicbb(4) allow the direct access of the OFW parent node by its children,
so they can be directly attached to iicbb(4) node on the DTS without the
need of describing the i2c bus.

r261845:
Allow the use of the OFW GPIO bus for ti_gpio and bcm2835_gpio.  With this
change the gpio children can be described as directly connected to the GPIO
controller without the need of describing the OFW GPIO bus itself on the
DTS file.

With this commit the OFW GPIO bus is fully functional on BBB and RPi.

GPIO controllers which want to use the OFW GPIO bus will need similar
changes.

r261846:
Make the gpioled(4) work out of the box on BBB.

Add gpioled(4) to BEAGLEBONE kernel and add the description of the four
on-board leds of beaglebone-black to its DTS file.

r262194:
Remove an unnecessary header.

r262522:
Fix make depend for iicbus.

r262559:
Inspired by r262522, fix make depend.  This fixes the build of gpio modules.
MFC r266092

gpioiic.4 and gpioled.4 will first appear in 10.1-RELEASE.