Currently, the compatible 'elgin,jg10309-01' is documented inside
trivial-devices.yaml, but it does not fit well there as it requires
extra properties such as spi-max-frequency, spi-cpha, and spi-cpol.

This causes the following dt-schema warnings:

make CHECK_DTBS=y rockchip/rv1108-elgin-r1.dtb -j12

  DTC [C] arch/arm/boot/dts/rockchip/rv1108-elgin-r1.dtb
rv1108-elgin-r1.dtb:display@0: 'spi-cpha', 'spi-cpol' do not match any of the regexes:
...

Fix this problem by introducing a specific binding for the Elgin
JG10309-01 SPI-controlled display.
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20240930213238.977833-1-festevam@gmail.comSigned-off-by: Rob Herring (Arm) <robh@kernel.org>