ARM: dts: imx27-phytec-phycore-som: Update FEC node

This patch updates FEC devicetree node of Phytec PCM038 module:
- Adding missing phy_mode properties.
- Adding fixed regulator to provide functionality without
  dummy-regulator in the kernel.

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