media: admin-guide: Add starfive_camss.rst for Starfive Camera Subsystem

Add starfive_camss.rst file that documents the Starfive Camera
Subsystem driver which is used for handing image sensor data.

Signed-off-by: Jack Zhu <jack.zhu@starfivetech.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>