[POWERPC] Add arch/powerpc support for the Motorola PrPMC2800

This finally adds the PPC_PRPMC2800 Kconfig option, the board setup
code (the setup and reset functions) and the defconfig, to support the
Motorola PrPMC2800 platform.

Signed-off-by: Dale Farnsworth <dale@farnsworth.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>