ppc: Initial HDEC support

The current behaviour isn't completely right, as for the DEC, we
don't properly re-arm when wrapping around, but I will fix this
in a separate patch.

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>
