drm: mxsfb: Add optional DPI output bus-width configuration

LCDIF programs LCD_DATABUS_WIDTH from the selected media bus format. The
format reported by the downstream panel or bridge describes the display
input, but it does not describe how the LCDIF data pins are physically
wired on the board.

These can differ. For example, a 16-bit LCDIF bus can be connected to a
24-bit display by wiring the available color bits to the corresponding
display inputs. In that case, using the display's 24-bit format to
configure LCDIF selects the wrong data-bus mode and changes the assignment
of color bits on the LCD_DATA pins.

Read the optional bus-width endpoint property from the LCDIF output port
and use it to select the media bus format used to configure LCDIF. This
allows the LCDIF bus mode to describe the physical interface
independently of the downstream display format.

When the property is absent, continue using the format reported by the
downstream display device, preserving the previous behavior. The
devicetree binding restricts bus-width to 16, 18 or 24; any other value
means the devicetree is broken, so fail the probe instead of silently
falling back to the previous behavior.

Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Tested-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Link: https://patch.msgid.link/20260917140042.118181-3-francesco@dolcini.it
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>

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