sim: gpio: update mask a/b signals better

sim: gpio: update mask a/b signals better

When the mask a/b MMRs are written, the output signal might change levels
(as pins are [un]masked), so make sure we update the output level.

Further, make sure we handle edge ints correctly by first sending a high
signal followed by a low signal.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>