drm: renesas: rz-du: mipi_dsi: Add LPCLK clock support

drm: renesas: rz-du: mipi_dsi: Add LPCLK clock support

Add LPCLK clock handling to the RZ/G2L MIPI DSI driver to support proper
DSI timing parameter configuration on RZ/V2H SoCs. While lpclk is present
on both RZ/G2L and RZ/V2H SoCs, the RZ/V2H SoC specifically uses the lpclk
rate to configure the DSI timing parameter ULPSEXIT.

Introduce a new lpclk field in the rzg2l_mipi_dsi structure and acquire
the "lpclk" clock during probe to enable lpclk rate-based timing
calculations on RZ/V2H while maintaining compatibility with RZ/G2L.

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: Tomi Valkeinen <tomi.valkeinen+renesas@ideasonboard.com>
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Link: https://patch.msgid.link/20251015192611.241920-7-prabhakar.mahadev-lad.rj@bp.renesas.com