[POWERPC] Add support for briq front panel

This adds the driver for the Briq front panel. This is a cleaned up
version of a driver that has been floating around for some time now,
initially written by Karsten Jeppesen <karsten@jeppesens.com> and
cleaned up by jk and myself.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>