watchdog: Add Cadence WDT driver

Add Cadence WDT driver. This is used by Xilinx Zynq.

Signed-off-by: Harini Katakam <harinik@xilinx.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>