staging: mt7621-gpio: avoid custom irq_domain for gpio

staging: mt7621-gpio: avoid custom irq_domain for gpio

Instead of create a custom irq_domain for this chip, use
'gpiochip_set_chained_irqchip' from GPIOLIB_IRQCHIP. It
is ok to call this function several times. We have to
manually mark the line with 'IRQF_SHARED' and then loop
over the three banks until you find a hit. There were
some problems with removing an irqchip like that but this
driver is a bool so it might work just fine. After this
changes the functions 'mediatek_gpio_to_irq' is not needed
anymore and also the 'gpio_irq_domain' field from the state
container. Instead of use the custom irq domain in the irq
handler use the associated domain from the gpio_chip in
'irq_find_mapping' function. Function 'mediatek_gpio_bank_probe'
has been moved a it to the botton to have all the irq related
functions together and avoid some forward declarations to resolve
some symbols along the code.

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