powerpc/pseries: move process table registration away from hash-specific code

This reduces ifdefs in a later change which makes hash support configurable.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211201144153.2456614-6-npiggin@gmail.com