[POWERPC] MPIC: support more than 256 sources

Allow more than the default 256 MPIC sources. Allocates a new flag
(MPIC_LARGE_VECTORS) to be used by platform code when instantiating
the mpic.

I picked 11 bits worth right now since it would cover the number of
sources on any hardware I have seen. It can always be increased later
if needed.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>