ARM: dts: imx6: RIoTboard provide standby on power off option

This board, as well as some other boards with i.MX6 and a PMIC, uses a
"PMIC_STBY_REQ" line to notify the PMIC about a state change.
The PMIC is programmed for a specific state change before triggering the
line.
In this case, PMIC_STBY_REQ can be used for stand by, sleep
and power off modes.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>