drm: renesas: rz-du: mipi_dsi: Add feature flag for 16BPP support

Introduce the `RZ_MIPI_DSI_FEATURE_16BPP` flag in `rzg2l_mipi_dsi_hw_info`
to indicate support for 16BPP pixel formats. The RZ/V2H(P) SoC supports
16BPP, whereas this feature is missing on the RZ/G2L SoC.

Update the `mipi_dsi_host_attach()` function to check this flag before
allowing 16BPP formats. If the SoC does not support 16BPP, return an error
to prevent incorrect format selection.

This change enables finer-grained format support control for different
SoC variants.

Co-developed-by: Fabrizio Castro <fabrizio.castro.jz@renesas.com>
Signed-off-by: Fabrizio Castro <fabrizio.castro.jz@renesas.com>
Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Link: https://lore.kernel.org/r/20250609225630.502888-8-prabhakar.mahadev-lad.rj@bp.renesas.com