sh: Stub out legacy PCC pgprot encoding for X2 TLBs.

Presently it's still possible to reference the PAGE_KERNEL_PCC pgprot
encoding on X2 TLBs which simply don't support it at all. Convert this
over to follow the nommu behaviour and simply hand back an invalid pgprot
value so we error out properly.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>