With real-time preemption, regmap functions cannot be used in the
implementation of irq_chip since they use spinlocks which may sleep.

Move the setting of the mux for IRQs to an irq_bus_sync_unlock handler
where we are allowed to sleep.
Signed-off-by: John Keeping <john@metanate.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>