ppc/hash32: Rework R and C bit updates

With MT-TCG, we are now running translation in a racy way, thus
we need to mimic hardware when it comes to updating the R and
C bits, by doing byte stores.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20190411080004.8690-5-clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>