dt-bindings: usb: renesas_gen3: Rename bindings documentation file to reflect IP block

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,usb3.txt to renesas,usb3-peri.txt

This refines a recent rename from renesas_usb3.txt to renesas,usb3.txt.
The motivation is to more accurately reflect the IP block documented in
this file.
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Sderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/20190809213710.31783-1-horms+renesas@verge.net.auSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>