dt-bindings: display: panel-timing: Define a single type for properties

It's not good practice to define multiple types for the same property, so
factor out the type reference making the properties always an uint32-array
with a length of 1 or 3 items.

Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://lore.kernel.org/r/20220413140016.3131013-1-robh@kernel.org