dt-bindings: display: add bindings for MIPI DBI compatible SPI panels

Add binding for MIPI DBI compatible SPI panels.

v6:
- Fix indentation (Rob)

v5:
- Add sainsmart18 to compatible items (Rob)
- Expand write-only description (Sam)

v4:
- There should only be two compatible (Maxime)
- s/panel-dbi-spi/panel-mipi-dbi-spi/in compatible

v3:
- Move properties to Device Tree (Maxime)
- Use contains for compatible (Maxime)
- Add backlight property to example
- Flesh out description

v2:
- Fix path for panel-common.yaml
- Use unevaluatedProperties
- Drop properties which are in the allOf section
- Drop model property (Rob)

Acked-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220227124713.39766-2-noralf@tronnes.org