ARM: dts: imx6qdl-udoo: add 7 inch LCD touchscreen panel support

The official UDOO board kit has 7 and 15.6 inch touchscreen LCD panels
as options.

This patch adds support for 7 inch panel only, but the 15.6 inch one
should be easy to add using the same regulator, backlight device and
LVDS channel.

Since this panel is an option for UDOO board it is disabled by default
and can be enabled (for example) by the following U-Boot commands:
fdt set backlight status okay
fdt set panelchan status okay
fdt set panel7 status okay
fdt set touchscreenp7 status okay

The LVDS channels is also disabled by default to avoid warning from its
driver.

Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>