dt-bindings: usb: renesas_gen3: Rename bindings documentation file

For consistency with the naming of (most) other documentation files for DT
bindings for Renesas IP blocks rename the Renesas USB3.0 peripheral
documentation file from renesas-gen3.txt to renesas,gen3.txt.
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>