powerpc: Merge the RELOCATABLE config entries for ppc32 and ppc64

It makes no sense to keep two separate RELOCATABLE config entries for
ppc32 and ppc64 respectively. Merge them into one and move it to a
common place.

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>