Add book3s.c

This adds the book3s core handling file. Here everything that is generic to
desktop PowerPC cores is handled, including interrupt injections, MSR settings,
etc.

It basically takes over the same role as booke.c for embedded PowerPCs.
Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>