dt-bindings: soc: loongson,ls2k-pmc: Use fallbacks for ls2k-pmc compatible

The Loongson-2K series chips (ls2k0500/ls2k1000/ls2k2000) share the same
PM system controller, using ls2k0500 compatible as fallback for the
others.

Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>