Convert to use threaded IRQs to support GPIOs that can sleep.
Protect the irq handler with mutex as it can be triggered from
two different irq lines accessing the same state.

This allows using GPIO expanders behind I2C or SPI bus.
Signed-off-by: Timo Teräs <timo.teras@iki.fi>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>