ppc: Build in all three of powermac, PREP and CHRP support

This reverts commit da0825fd, making
it so that if you select CONFIG_PPC_MULTIPLATFORM you get support
for PMAC, PREP and CHRP built in.

The reason for not allowing PMAC, PREP and CHRP to be selected
individually for ARCH=ppc is that there is too much interdependency
between them in the platform support code.  For example, CHRP uses
the PMAC nvram code.

Configuring with ARCH=powerpc does allow you to select support for
PMAC and CHRP separately.  Support for PREP is not there yet but
should be there soon.

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