ARM: dts: imx6: phycore-som: add pmic onkey device

Without the onkey device it isn't possible to power off the system using
the X_PMIC_nONKEY signal which is routed to the SoM pin header.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>