dt-bindings: rtc: loongson: Document Loongson-2K0300 compatible

dt-bindings: rtc: loongson: Document Loongson-2K0300 compatible

Add "loongson,ls2k0300-rtc" dedicated compatible to represent the RTC
interface of the Loongson-2K0300 chip.

Its hardware design is similar to that of the Loongson-1B, but it does
not support the alarm feature.

Reviewed-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/0c6a4fd221b5bf05f06cae9bb543a5bf7ea59f57.1768616276.git.zhoubinbin@loongson.cn
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>