media: rzg2l-cru: Manually track active slot number

The CRU cycles over the hardware slots where the destination address for
the next frame has to be programmed.

The RZ/G2L version of the IP has a register that tells which is the
last used slot by the hardware but, unfortunately, such register is not
available on RZ/G3E and RZ/V2H(P).

The driver currently compares the value of the AMnMADRSL/H register
which report "the memory address which the current video data was
written to" and compares it with the address programmed in the slots.

This heuristic requires a bit of book keeping and proper locking. As the
driver handles the FrameEnd interrupt, it's way easier to keep track
of the slot that has been used by ourselves with a driver variable.

Signed-off-by: Jacopo Mondi <jacopo.mondi+renesas@ideasonboard.com>
Tested-by: Tommaso Merciai <tommaso.merciai.xr@bp.renesas.com>
Reviewed-by: Tommaso Merciai <tommaso.merciai.xr@bp.renesas.com>
Reviewed-by: Daniel Scally <dan.scally@ideasonboard.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>

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