dt-bindings: rtc: Split loongson,ls2x-rtc into SoC-based compatibles

Move Loongson RTC bindings from trivial-rtc.yaml into loongson,rtc.yaml.

The architectures associated with this driver use the built-in DTB, so
we can just drop the compatible(rtc-ls2x) with wildcards.
Also, soc-based compatible is more accurate for hardware differences
between chips.

Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/1b0bb443bd74647c17b7902f3d719700f81a1dba.1685693501.git.zhoubinbin@loongson.cn


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>