rtc: loongson: Add Loongson-2K0300 support

rtc: loongson: Add Loongson-2K0300 support

The Loongson-2K0300's rtc hardware design is similar to that of the
Loongson-1B, but it does not support the alarm feature.

Introduce `LOONGSON_RTC_ALARM_WORKAROUND`, which indicates a chip that
does not support the alarm feature, and rewrite the related logic in
`loongson_rtc_alarm_setting()`.

Reviewed-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
Link: https://patch.msgid.link/abff68dda2fe6a6601a9e58b31e278d941297fce.1768616276.git.zhoubinbin@loongson.cn
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>