ARM: dts: imx28: move lcdif control pins into board dts

The presence and muxing of lcdif control pins can vary much on different
boards.  Move them into imx28-evk.dts to make lcdif_24bit_pins_a only
about data pins, so that it can be reused by other boards.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>