powerpc: move find_and_init_phbs() to pSeries specific code

Previously, find_and_init_phbs() was used in both PowerNV and pSeries
setup. However, since RTAS support has been dropped from PowerNV, we
can move it into a platform-specific file.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>