target/ppc: Add a function to check for page protection bit

target/ppc: Add a function to check for page protection bit

Checking if a page protection bit is set for a given access type is a
common operation. Add a function to avoid repeating the same check at
multiple places. As this relies on access type and page protection bit
values having certain relation also add an assert to ensure that this
assumption holds.

Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>