watchdog: davinci: use davinci_wdt_device structure to hold device data

Some SoCs, like Keystone 2, can support more than one WDT and each
watchdog device has to use it's own base address, clock source,
watchdog device, so add new davinci_wdt_device structure to hold
device data.

Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Reviewed-by: Guenter roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>