arm: mx6: cm-fx6: add support for displaytype env var

arm: mx6: cm-fx6: add support for displaytype env var

Add support for selecting display preset using the environment variable
"displaytype". This is a preparation for future merging of compulab
omap3_display.c display selection code with the cm-fx6 display selection code.

The "panel" environment variable is retained for backwards compatibility.

Cc: Stefano Babic <sbabic@denx.de>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>