soc: loongson: loongson_pm2: Drop useless of_device_id compatible

Now, "loongson,ls2k0500-pmc" is used as fallback compatible, so the
ls2k1000 compatible in the driver can be dropped directly.

Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>