media: rcar-fcp: Add rcar_fcp_soft_reset()

Add a function to perform soft reset of the FCP.

It is intended to support the correct stop procedure of the VSPX-FCPVX
and VSPD-FCPD pairs according to section "62.3.7.3 Reset Operation" of
the R-Car Hardware Manual at revision 1.20.

Signed-off-by: Jacopo Mondi <jacopo.mondi+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Link: https://lore.kernel.org/r/20250616-vspx-reset-v2-1-6cc12ed7e9bb@ideasonboard.com


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