watchdog: uniphier: add UniPhier watchdog driver

Add a watchdog driver for Socionext UniPhier series SoC.
Note that the timeout value for this device must be a power
of 2 because of the specification.

Signed-off-by: Keiji Hayashibara <hayashibara.keiji@socionext.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>