dt-bindings: rcar-imr: Rename bindings documentation file

Renesas media binding documentation files use a naming schema of
'renesas,<module>.txt'. Rename the IMR file to match this pattern.
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>