dt-bindings: watchdog: mt7621-wdt: add phandle to access system controller registers

dt-bindings: watchdog: mt7621-wdt: add phandle to access system controller registers

MT7621 SoC provides a system controller node for accessing to some registers.
Add a phandle in this node to avoid using MIPS related arch operations and
includes in watchdog driver code.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20230214103936.1061078-2-sergio.paracuellos@gmail.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>