leds: Teach leds-gpio to handle timer-unsafe GPIOs

Teach the new leds-gpio driver that some GPIOs can't be accessed from
timer callbacks ... which is how all today's standard triggers use them.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Richard Purdie <rpurdie@rpsys.net>