MIPS: Add multiplatform BMIPS target

bmips_be_defconfig supports Linux running on the following CM and DSL
SoCs:

 - BCM3384 (BMIPS5000) cable modem application processor, BE, SMP
 - BCM3384 (BMIPS4355) cable modem "spare CPU"*, BE
 - BCM6328 (BMIPS4355) ADSL chip, BE
 - BCM6368 (BMIPS4350) ADSL chip, BE, SMP

*experimental; most configurations will require changing CONFIG_PHYSICAL_START

bmips_stb_defconfig supports Linux running on the (nominally LE) STB
chipsets:

 - BCM7125 (BMIPS4380) set-top box chip, LE, SMP
 - BCM7346 (BMIPS5000) set-top box chip, LE, SMP
 - BCM7360 (BMIPS3300) set-top box chip, LE
 - BCM7420 (BMIPS5000) set-top box chip, LE, SMP
 - BCM7425 (BMIPS5000) set-top box chip, LE, SMP

serial8250 and bcm63xx_uart do not currently coexist.  If/when this is
fixed, it will be also possible to boot the BE image on any supported STB
board configured for BE.  For now, each defconfig can only pick one UART
driver, and the BE defconfig enables bcm63xx_uart.

On these MIPS systems, endianness cannot be reconfigured at runtime.  On
STB it is sometimes offered as a board jumper or 0-ohm resistor, and
sometimes hardwired to LE only.  The CM and DSL systems always run BE.

Device Tree is used to configure the following items:

 - UART, USB, GENET peripherals
 - IRQ controllers
 - Early console base address (bcm63xx_uart only)
 - SMP or UP mode
 - MIPS counter frequency
 - Memory size / regions
 - DMA remappings
 - Kernel command line

The DT-enabled bootloader and build instructions for 3384 are posted at
https://github.com/Broadcom/aeolus .  The other chips use legacy non-DT
bootloaders, and so the kernel employs autodetection heuristics to select
a builtin DTB.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>