media: rcar-csi2: Add warning for PHY speed less than minimum

Add a warning message when the selected PHY speed is less
than supported minimum PHY speed given in the hsfreq table[1].

For raspberry pi camera capture on Kingfisher board with resolution
640x480, the calculated PHY speed is 48 mbps which is less than
the minimum PHY speed 80 Mbps from the table[1]. But in this cases
capture is successful.

[1] specs r19uh0105ej0200-r-car-3rd-generation.pdf [Table 25.9]

Signed-off-by: Suresh Udipi <sudipi@jp.adit-jv.com>
Signed-off-by: Michael Rodin <mrodin@de.adit-jv.com>
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+huawei@kernel.org>