dt-bindings: define vendor prefix for whwave, Inc.

Introduce vendor prefix for whwave, Inc.
for SD3078 rtc device.

Signed-off-by: Dianlong Li <long17.cool@163.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>