The access to the internal storage of the format rcar_csi2.mf should be
serialized, extend the existing lock mutex to also cover this.

While at it document the mutex.
Signed-off-by: NNiklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: NJacopo Mondi <jacopo@jmondi.org>
Reviewed-by: NKieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: NHans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: NMauro Carvalho Chehab <mchehab+huawei@kernel.org>