Replace the custom IRQ chip handler and mask/unmask functions with
REGMAP_IRQ. This significantly simplifies the code and allows for the
removal of almost all interrupt-related functions from mt7530.c.
Tested on MT7988A built-in switch (MMIO) as well as MT7531AE IC (MDIO).
Signed-off-by: Daniel Golle <daniel@xxxxxxxxxxxxxx>