ppc: Use a helper to filter writes to LPCR

This handles filtering bits based on what is implemented by a
given architecture version. We also use it to copy to LPCR
some of the relevant 970 HID4 bits.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
[clg: fixed checkpatch.pl errors ]
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
