gpio: mt7621: Switch to use platform_get_irq() function

Mt7621 SoC GPIO driver is a platform driver so we can directly use
'platform_get_irq' instead of 'irq_of_parse_and_map'.

Tested-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>