drm/bridge: lt9611: Add support for single Port B input

drm/bridge: lt9611: Add support for single Port B input

The LT9611 has two DSI input ports. The driver currently assumes Port A
is always used for single-port configurations. However, some boards
connect DSI to Port B only.

Update the driver to detect which ports are populated from devicetree
and configure the hardware accordingly:

- If only port@1 (Port B) is populated, configure port swap (0x8303
  bit 6) and byte_clk source (0x8250 bit 3:2) for Port B operation
- If both ports are populated, use dual-port mode (Port A + B)
- If only port@0 (Port A) is populated, use single Port A (existing
  behavior)

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Signed-off-by: Hongyang Zhao <hongyang.zhao@thundersoft.com>
Reviewed-by: Roger Shimizu <rosh@debian.org>
Tested-by: Roger Shimizu <rosh@debian.org>
Link: https://patch.msgid.link/20260207-rubikpi-next-20260116-v3-2-23b9aa189a3a@thundersoft.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>