ARM: dts: imx27-phytec-phycore-som: Disable PM pins for USB OTG

USB PWR and OC pins are used as GPIOs for different purposes,
so add "disable-over-current" property for OTG node to indicate this.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>