drm/rcar-du: dsi: Implement DSI command support

drm/rcar-du: dsi: Implement DSI command support

Implement support for DSI command transfer. Transmission of both Short
Packet and Long Packet is implemented, so is command transmission to
request response from peripheral device and transmission of non-read
command with BTA.

The AXI memory access mode is currently not implemented, each transfer
is performed purely using controller register interface. Short Packet
transfer can transfer up to 2 Bytes of data, Long Packet transfer can
transfer up to 16 Bytes of data.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Link: https://lore.kernel.org/r/20250831190507.327848-1-marek.vasut+renesas@mailbox.org
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>