dt-bindings: rtc: Add Loongson LS2X RTC support

Document the binding for the LS2X RTC block found on the Loongson-2K SoC
and the LS7A bridge, originally appearing on the Loongson-2H.
Signed-off-by: WANG Xuerui <git@xen0n.name>
Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/0288efeb4209e4a49af07de6399045aaa00a970c.1673227292.git.zhoubinbin@loongson.cnSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

Documentation/devicetree/bindings/rtc/trivial-rtc.yaml

@@ -47,6 +47,8 @@ properties:
       - isil,isl1218
       # Intersil ISL12022 Real-time Clock
       - isil,isl12022
+      # Loongson-2K Socs/LS7A bridge Real-time Clock
+      - loongson,ls2x-rtc
       # Real Time Clock Module with I2C-Bus
       - microcrystal,rv3028
       # Real Time Clock Module with I2C-Bus