media: rcar-isp: Rename base register variable

media: rcar-isp: Rename base register variable

Prepare for extending the driver to in addition to supporting the
channel selector (CS) also support the core ISP. The two different
functions have different base addresses so the driver needs to
distinguish between them.

Prepare for this by marking existing base address variable and
read/write functions to make it clear they operate on the CS portion of
the driver. There is no functional change.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://lore.kernel.org/r/20250423163113.2961049-7-niklas.soderlund+renesas@ragnatech.se
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>