MIPS: JZ4740: dts: Add bindings for the jz4740-wdt driver

Also remove the watchdog platform_device from platform.c, since it
wasn't used anywhere anyway.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Wim Van Sebroeck <wim@linux-watchdog.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Malaterre <malat@debian.org>
Cc: linux-mips@linux-mips.org
Cc: linux-watchdog@vger.kernel.org
Cc: devicetree@vger.kernel.org
[jhogan@kernel.org: Drop jz4740_wdt_device declaration from header]
Signed-off-by: James Hogan <jhogan@kernel.org>