powerpc/83xx: Add support for MCU microcontroller in .dts files

MCU is an external Freescale MC9S08QG8 microcontroller, mainly used to
provide soft power-off function, but also exports two GPIOs (wired to
the LEDs and also available from the external headers).

Added the MCU on mpc8349emitx, mpc837xrdb and mpc8315erdb boards.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>