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.
1061078-2-sergio.paracuellos@gmail.com
Tested on Motorola MWR03 (MT7628)
Signed-off-by: Shiji Yang <yangshiji66@qq.com>