drivers: core: ofnode: fix typo in panel timing decode

In case where a single timing resolution is implemented in the
device-tree, the property is named "panel-timing", as specify
in Linux kernel binding file:

Documentation/devicetree/bindings/display/panel/panel-common.yaml

  # Display Timings
  panel-timing:
    description:
      Most display panels are restricted to a single resolution and
      require specific display timings. The panel-timing subnode expresses those
      timings.
    $ref: panel-timing.yaml#

  display-timings:
    description:
      Some display panels support several resolutions with different timings.
      The display-timings bindings supports specifying several timings and
      optionally specifying which is the native mode.
    $ref: display-timings.yaml#

Fixes: 0347cc77 ("drivers: core: ofnode: Add panel timing decode.")
Signed-off-by: Raphael Gallais-Pou <raphael.gallais-pou@foss.st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>