nitrogen6x: display: add wvga-lvds panel

Add support for WVGA (800x480) panels using VESA GTF timings over
LVDS.

No auto-detection is supported, so you must configure this panel
manually through the 'panel' environment variable:

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

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