dt-bindings: display: panel: mipi-dbi-spi: Add a pixel format property

The MIPI DBI 2.0 specification (2005) lists only two pixel formats for
the Type C Interface (SPI) and that is 3-bits/pixel RGB111 with
2 options for bit layout.

For Type A and B (parallel) the following formats are listed: RGB332,
RGB444, RGB565, RGB666 and RGB888 (some have 2 options for the bit layout).

Many MIPI DBI compatible controllers support all interface types on the
same chip and often the manufacturers have chosen to provide support for
the Type A/B interface pixel formats also on the Type C interface.

Some chips provide many pixel formats with optional bit layouts over SPI,
but the most common by far are RGB565 and RGB666. So even if the
specification doesn't list these formats for the Type C interface, the
industry has chosen to include them.

The MIPI DCS specification lists the standard commands that can be sent
over the MIPI DBI interface. The set_address_mode (36h) command has one
bit in the parameter that controls RGB/BGR order:
    This bit controls the RGB data latching order transferred from the
    peripheral’s frame memory to the display device.
This means that each supported RGB format also has a BGR variant.

Based on this rationale document the following pixel formats describing
the bit layout going over the wire:
- RGB111 (option 1): x2r1g1b1r1g1b1 (2 pixels per byte)
- BGR111 (option 1): x2b1g1r1b1g1r1 (2 pixels per byte)
- RGB111 (option 2): x1r1g1b1x1r1g1b1 (2 pixels per byte)
- BGR111 (option 2): x1b1g1r1x1b1g1r1 (2 pixels per byte)
- RGB565: r5g6b5 (2 bytes)
- BGR565: b5g6r5 (2 bytes)
- RGB666: r6x2g6x2b6x2 (3 bytes)
- BGR666: b6x2g6x2r6x2 (3 bytes)
(x: don't care)

v2:
- Use 'default: r5g6b5' (Rob)

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240604-panel-mipi-dbi-rgb666-v4-1-d7c2bcb9b78d@tronnes.org


Signed-off-by: Noralf Trønnes <noralf@tronnes.org>