Use immediate offsets rather than post-increment

When we set the GICD_IGROUPRn registers, we have a dangling
post-increment at the end which is never useful, being overwritten in
every path. As we're only writing to 3 registers, the offsets of which
can be represented in immediates, use movs with immediate offsets to
perform the writes.

Reported-by: Nigel Stephens <nigel.stephens@arm.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>