drm/tiny: Add MIPI DBI compatible SPI driver

Add a driver that will work with most MIPI DBI compatible SPI panels.
This avoids adding a driver for every new MIPI DBI compatible controller
that is to be used by Linux. The 'compatible' Device Tree property with
a '.bin' suffix will be used to load a firmware file that contains the
controller configuration.

Example (driver will load sainsmart18.bin):

display@0 {
	compatible = "sainsmart18", "panel-mipi-dbi-spi";
...
};

v5:
- kconfig: s/DRM_KMS_CMA_HELPER/DRM_GEM_CMA_HELPER/ (Sam)
- kconfig: Add select VIDEOMODE_HELPERS (Sam)
- kconfig: Add wiki url in the description (Sam)
- Split out and use of_get_drm_panel_display_mode()(Sam)
- Only use the first compatible to look for a firmware file since the
  binding mandates 2 compatibles.
- Make having a firmware file mandatory so we can print an error
  message if it's missing to improve the user experience. It's very
  unlikely that a controller doesn't need to be initialized and if
  it doesn't, it's possible to have a firmware file containing only
  a DCS NOP.

v4:
- Move driver to drm/tiny where the other drivers of its kind are located.
  The driver module will not be shared with a future DPI driver after all.

v3:
- Move properties to DT (Maxime)
- The MIPI DPI spec has optional support for DPI where the controller is
  configured over DBI. Rework the command functions so they can be moved
  to drm_mipi_dbi and shared with a future panel-mipi-dpi-spi driver

v2:
- Drop model property and use compatible instead (Rob)
- Add wiki entry in MAINTAINERS

Acked-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220227124713.39766-6-noralf@tronnes.org