media: rcar-csi2: Add helper to lookup mbps settings

media: rcar-csi2: Add helper to lookup mbps settings

The structure mapping a configuration information to a particular mpbs
setting needs to be extended with more information to support future
SoCs. Before it is extended reduce code duplication by creating a helper
to lookup information from an array of mbps setting, the lookup code
has already been copied to two speared locations.

While at it rename the structure to make it clear it contains
information related to a mbps setting, not just a single register value.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>