drm: renesas: rz-du: mipi_dsi: Add function pointers for configuring VCLK and mode validation

Introduce `dphy_conf_clks` and `dphy_mode_clk_check` callbacks in
`rzg2l_mipi_dsi_hw_info` to configure the VCLK and validate
supported display modes.

On the RZ/V2H(P) SoC, the DSI PLL dividers need to be as accurate as
possible. To ensure compatibility with both RZ/G2L and RZ/V2H(P) SoCs,
function pointers are introduced.

Modify `rzg2l_mipi_dsi_startup()` to use `dphy_conf_clks` for clock
configuration and `rzg2l_mipi_dsi_bridge_mode_valid()` to invoke
`dphy_mode_clk_check` for mode validation.

This change ensures proper operation across different SoC variants
by allowing fine-grained control over clock configuration and mode
validation.

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>
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Link: https://lore.kernel.org/r/20250609225630.502888-10-prabhakar.mahadev-lad.rj@bp.renesas.com