ramips: mt7621-wdt: use phandle to access system controller registers

ramips: mt7621-wdt: use phandle to access system controller registers

These patches allow the driver to access some watchdog registers via a
phandle to the system controller node[1]. To apply these changes, we
need to add "mediatek,sysctl" to the SoC dtsi. This commit also remove
the redundent clocks, interrupts and resets properties.

[1] https://lore.kernel.org/all/20230214103936.1061078[email protected]

Tested on Motorola MWR03 (MT7628)

Signed-off-by: Shiji Yang <[email protected]>