drm/solomon: Add SSD130x OLED displays SPI support

The ssd130x driver only provides the core support for these devices but it
does not have any bus transport logic. Add a driver to interface over SPI.

There is a difference in the communication protocol when using 4-wire SPI
instead of I2C. For the latter, a control byte that contains a D/C# field
has to be sent. This field tells the controller whether the data has to be
written to the command register or to the graphics display data memory.

But for 4-wire SPI that control byte is not used, instead a real D/C# line
must be pulled HIGH for commands data and LOW for graphics display data.

For this reason the standard SPI regmap can't be used and a custom .write
bus handler is needed.

Signed-off-by: Javier Martinez Canillas <[email protected]>
Acked-by: Mark Brown <[email protected]>
Reviewed-by: Geert Uytterhoeven <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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