powerpc/rtas: Move post_mobility_fixup() declaration to pseries

This is a pseries-specific function declaration that doesn't belong in
rtas.h. Move it to the pseries platform code and adjust
pseries/suspend.c accordingly.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231106-rtas-trivial-v1-5-61847655c51f@linux.ibm.com