mmu-hash32: Cleanup BAT lookup

This patch makes a general cleanup of the ppc_hash32_get_bat() function,
renaming it to ppc_hash32_bat_lookup().  In particular, the new function
only looks for a matching BAT, with the permissions check from the old
function moved to the caller.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>
