ramips: ralink-gpio: use irqchip helpers to register driver

ramips: ralink-gpio: use irqchip helpers to register driver

The gpiolib has already introduced a general GPIO irqchip framework
to initialize the GPIO irqchip[1]. This patch will make use of it
to simplify the legacy Ralink GPIO driver codes. This patch also
includes some code readability improvements.

[1] 1425052097b5 ("gpio: add IRQ chip helpers in gpiolib")
Signed-off-by: Shiji Yang <yangshiji66@qq.com>
Link: https://github.com/openwrt/openwrt/pull/16764
Signed-off-by: Robert Marko <robimarko@gmail.com>