powerpc/booke: Only use VIRT_PHYS_OFFSET on booke32

The way VIRT_PHYS_OFFSET is not correct on book3e-64, because
it does not account for CONFIG_RELOCATABLE other than via the
32-bit-only virt_phys_offset.

book3e-64 can (and if the comment about a GCC miscompilation is still
relevant, should) use the normal ppc64 __va/__pa.

At this point, only booke-32 will use VIRT_PHYS_OFFSET, so given the
issues with its calculation, restrict its definition to booke-32.

Signed-off-by: Scott Wood <scottwood@freescale.com>