mmu-hash32: Clean up BAT matching logic

The code to search for a matching BAT for a virtual address is somewhat
longwinded and awkward.  In particular, it relies on seperate size and
validity information being returned from the hash32_bat_size() function
(and 601 specific variant).

We simplify this by having hash32_bat_size() return instead a mask of the
virtual address bits to match, and 0 for invalid (since a BAT can never
match the entire address space).

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