media: adv748x: csi2: Link AFE with TXA and TXB

The ADV748x chip supports routing AFE output to either TXA or TXB.
In order to support run-time configuration of video stream path, create an
additional (not enabled) "AFE:8->TXA:0" link, and remove the IMMUTABLE flag
from existing ones.

Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>