ofnode: support panel-timings in ofnode_decode_display_timing

ofnode: support panel-timings in ofnode_decode_display_timing

The "Display Timings" in panel-common.yaml can be provided by 2 properties
- panel-timing: when display panels are restricted to a single resolution
                the "panel-timing" node expresses the required timings.
- display-timings: several resolutions with different timings are supported
                   with several timing subnode of "display-timings" node

This patch update the parsing function to handle this 2 possibility
when index = 0.

Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Yannick Fertre <yannick.fertre@foss.st.com>
Signed-off-by: Raphael Gallais-Pou <raphael.gallais-pou@foss.st.com>