powerpc/ppc-opcode: Add PPC_RAW_MFSPR()

Add PPC_RAW_MFSPR() to replace open coding done in 8xx-pmu.c

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/e281e3a611eead8817c49cf06a60072a021af823.1606231483.git.christophe.leroy@csgroup.eu