arm: mx6: cm-fx6: add splash screen support

Add support for splash screen.
The splash screen is loaded from the SPI flash and is displayed on the
HDMI display.

Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
[grinberg@compulab.co.il: minor code and commit message updates]
Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Igor Grinberg <grinberg@compulab.co.il>