selftests/powerpc: Update PROT_SAO test to skip ISA 3.1

Since SAO support was removed from ISA 3.1, skip the
prot_sao test if PPC_FEATURE2_ARCH_3_1 is set.

Signed-off-by: Shawn Anastasio <shawn@anastas.io>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200821185558.35561-4-shawn@anastas.io