dt-bindings: watchdog: Rename bindings documentation file

For consistency with the naming of (most) other documentation files for DT
bindings for Renesas IP blocks rename the Renesas WDT documentation file
from renesas-wdt.txt to renesas,wdt.txt.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>