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

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