nitrogen6x: display: add svga display (800x600)
Add support for 800x600 18-bit RGB displays using VESA GTF timings.
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>