dt-bindings: display: convert ilitek,ili9341.txt to dt-schema

dt-bindings: display: convert ilitek,ili9341.txt to dt-schema

A dt-schema binding for the Ilitek ili9341 was created as
panel/ilitek,ili9341.yaml but the txt binding was ignored in the
process. Move the remaining items in the txt binding to the yaml one &
delete it.

The example in the txt binding has a spi-max-frequency which disagrees
with the yaml replacement (and its own documentation) so change that to
conform with the binding. There are no users in tree of the Adafruit
yx240qv29 to check against.

Link: https://cdn-learn.adafruit.com/assets/assets/000/046/879/original/SPEC-YX240QV29-T_Rev.A__1_.pdf
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20220705215213.1802496-2-mail@conchuod.ie
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>