powerpc: Fix branch patching code for ABIv2

The MMU hashtable and SLB branch patching code uses function
pointers for the update sites. This creates a difference between
ABIv1 and ABIv2 because we don't have function descriptors on
ABIv2.

Get rid of the function pointer and just point at the update
sites directly. This works on both ABIs.

Signed-off-by: Anton Blanchard <anton@samba.org>