dt-bindings: rtc: mpfs-rtc: permit resets

The RTC on mpfs and pic64gx has a reset pin, but until now this has been
undocumented because platform firmware takes the RTC out of reset on
first-party boards (or those using modified versions of the vendor
firmware), but not all boards may take this approach. Permit providing a
reset in devicetree for Linux, or other devicetree-consuming software,
to use.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Link: https://patch.msgid.link/20260303-flounder-slate-dd69766990ce@spud
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>