powerpc psim: Fix infinite recursion in multiple bus methods

Similar to 68a3ff041129208ea98a3bd5142061176ab4165e, the default case
needs to call bus_generic_* to pass the request up the tree, not bus_*
which will just call this method again.

Fixes: d7c16b333455 powerpc psim: Use bus_generic_rman_*
(cherry picked from commit 61d9bd21e9d6c3f876c4c8549fafaaaa4de75983)