Document: devicetree: input: imx: i.mx snvs power device tree bindings

The snvs-pwrkey is designed to enable POWER key function which controlled
by SNVS ONOFF. the driver can report the status of POWER key and wakeup
system if pressed after system suspend.

Signed-off-by: Frank Li <Frank.Li@freescale.com>
Signed-off-by: Robin Gong <b38343@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>