dt-bindings: rcar-{csi2,vin}: Rename bindings documentation files

Renesas media binding documentation files use a naming schema of
'renesas,<module>.txt'. Rename the VIN and CSI-2 files to match this
pattern.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Ulrich Hecht <uli+renesas@fpond.eu>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>