drivers: core: ofnode: Add panel timing decode.

ofnode_decode_display_timing supports reading timing parameters from
subnode of display-timings node, for displays supporting multiple
resolution, in case if a display supports single resolution, it fails
reading directly from display-timings node, to support it
ofnode_decode_panel_timing is added.

Signed-off-by: Nikhil M Jain <n-jain1@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>