[POWERPC] Add DTS file for the Motorola PrPMC2800 platform

Add the device tree source file for the prpmc2800 line of processor PMCs.
Several of the property values are updated by the bootwrapper but sane
defaults have been chosen in case the bootwrapper can't determine the
exact processor board variant.  The defaults should allow the kernel
to boot despite having non-optimal device tree property values.

Signed-off-by: Mark A. Greer <mgreer@mvista.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>