ppc: Do not take exceptions on unknown SPRs in privileged mode

The architecture specifies that mtspr/mfspr on an unknown SPR number
should act as a nop in privileged mode.

I haven't removed the warning however as it can be useful for
diagnosing.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
