nitrogen6x: display: add qvga panel

Add support for a 1/4 VGA panel with a 24-bit RGB interface.
No auto-detection is enabled, so you must configure the 'panel'
environment variable to use this display:

        U-Boot > setenv panel qvga
        U-Boot > saveenv && reset

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>