staging: mt7621-gpio: implement '.irq_[request|release]_resources' functions

When implementing custom irqchips it is important to also
implement .irq_request_resources() and .irq_release_resources()
and make sure these call gpiochip_[un]lock_as_irq().
Add those two for this driver. Also store struct device pointer
in global state structure to be able to use 'dev_err' with the
device from 'mediatek_request_resources' function.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>