ARM: mxs: Enable touchscreen on m28evk

This patch adds necessary DT properties to m28evk to enable touchscreen
in the LRADC block. The M28EVK is shipped with 4-wire resistive touchscreen.

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>