ppc: Fix support for odd MSR combinations

MacOS uses an architecturally illegal MSR combination that
seems nonetheless supported by 32-bit processors, which is
to have MSR[PR]=1 and one or more of MSR[DR/IR/EE]=0.

This adds support for it. To work properly we need to also
properly include support for PR=1,{I,D}R=0 to the MMU index
used by the qemu TLB.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Tested-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
