dt-bindings: mfd: rn5t618: Document optional property system-power-controller

The RN5T618 family of PMICs can be used as system management
controllers, in which case they handle poweroff and restart. Document
this capability by referring to the corresponding generic DT binding.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Lee Jones <lee.jones@linaro.org>