powerpc: Fix invalid construct in our CPU selection Kconfig

commit 5b7c3c91 introduced an invalid
construct in our CPU selection. This caused warnings, though it still
appeared to do the right thing.

This fixes it properly by having separate formal definitions of
PPC_BOOK3S_32 and PPC_BOOK3S_64 and one statement defining
PPC_BOOK3S based on the two above.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>