Documentation: dt: binding: fsl: add devicetree binding for describing RCPM

RCPM is the Run Control and Power Management module performs all
device-level tasks associated with device run control and power
management.

Add this for freescale powerpc platform and layerscape platform.

Signed-off-by: Chenhui Zhao <chenhui.zhao@freescale.com>
Signed-off-by: Tang Yuantian <Yuantian.Tang@freescale.com>
Signed-off-by: Wang Dongsheng <dongsheng.wang@freescale.com>
[scottwood: s/pointer/phandle and "disabled" status from example]
Signed-off-by: Scott Wood <oss@buserror.net>