drm: renesas: rz-du: mipi_dsi: Add dphy_late_init() callback for RZ/V2H(P)

Introduce the `dphy_late_init` callback in `rzg2l_mipi_dsi_hw_info` to
allow additional D-PHY register configurations after enabling data and
clock lanes. This is required for the RZ/V2H(P) SoC but not for the
RZ/G2L SoC.

Modify `rzg2l_mipi_dsi_startup()` to invoke `dphy_late_init` if defined,
ensuring SoC-specific initialization is performed only when necessary.

This change prepares for RZ/V2H(P) SoC support while maintaining
compatibility with existing platforms.

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/20250609225630.502888-9-prabhakar.mahadev-lad.rj@bp.renesas.com