watchdog: davinci: reuse driver for keystone arch

The keystone arch uses the same IP watchdog, so add "ti,keystone-wdt"
compatible and correct identity.

The Keystone arch is using clocks in DT and source clock for watchdog
has to be specified, so add this to binding.

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>