bmips: Add Inteno XG6846 board

This adds support for the Inteno XG6846 board based on the
Broadcom MIPS 6328 SoC.

The default boot will read a uImage from flash and boot it.

Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>