dt-bindings: soc: loongson,ls2k-pmc: Allow syscon-reboot/syscon-poweroff as child

The reboot and poweroff features are actually part of the Power
Management Unit system controller, thus allow them as its children,
instead of specifying as separate device nodes with syscon phandle.

Without it, the reboot/poweroff feature becomes unavailable.

Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>