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.

Approved by:	adrian (mentor, implicit)