dt-bindings: rtc: loongson: Correct Loongson-1C interrupts property

The `interrupts` property indicates an RTC alarm interrupt, which is
required for RTCs that support the alarm feature, which is not supported
by the Loongson-1C RTC. We exclude it for a more accurate description.

Changing the `allowed` property is ABI-breaking behavior, but
throughout the existing Loongson DTS{i}, the description of the RTC
nodes conforms to the modified bingding rules.

Thus, the existing Loongson DTS{i} will not be affected.

Fixes: 487ef32caebe ("dt-bindings: rtc: Split loongson,ls2x-rtc into SoC-based compatibles")
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/b6295c907410f6708115cba4df0959ee6629f8a5.1768616276.git.zhoubinbin@loongson.cn
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>