fdt: add new fdt_fixup_display function to configure display

Add 'fdt_fixup_display' function to fixup device-tree native-mode property
of display-timings node to select timings for a specific display.
This is useful if a device-tree has configurations for multiple
display timings for undetectable displays.

see kernel Documentation/devicetree/bindings/video/display-timing.txt

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Acked-by: Simon Glass <sjg@chromium.org>