target/ppc: add user read/write functions for MMCR2

Similar to the previous patch, let's add problem state read/write access to
the MMCR2 SPR, which is also a group A PMU SPR that needs to be filtered
to be read/written by userspace.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20211018010133.315842-4-danielhb413@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>