powerpc: set CONFIG_PPC_OF=y always for ARCH=powerpc

The CONFIG_PPC_OF symbol is used to mean that the firmware device tree
access functions are available.  Since we always have a device tree
with ARCH=powerpc, make CONFIG_PPC_OF always Y for ARCH=powerpc.

This fixes some compile errors reported by Kumar Gala, but in a
different way to his patch.  This also makes prom_parse.o be compiled
only if CONFIG_PPC_OF so that non-OF ARCH=ppc platforms will compile.

Signed-off-by: Paul Mackerras <paulus@samba.org>