ARM: dts: imx27-phytec-phycore-rdk: Add display control signal definitions

This patch adds display control signal definitions.
These fields are not used in the driver yet, but will be used for
reference to indicate the polarity of the signals.

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