media: rzg2l-cru: Add function pointer to check if FIFO is empty

Add a `fifo_empty` function pointer to the `rzg2l_cru_info` structure and
pass it as part of the OF data. On RZ/G3E and RZ/V2H(P) SoCs, checking if
the FIFO is empty requires a different register configuration.

Implement `rzg2l_fifo_empty()` and update the code to use it from the
function pointer.

Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Signed-off-by: Tommaso Merciai <tommaso.merciai.xr@bp.renesas.com>
Link: https://lore.kernel.org/r/20250411170624.472257-16-tommaso.merciai.xr@bp.renesas.com


Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>