watchdog: Add Xilinx watchdog timer driver

Watchdog timer device driver for Xilinx xps_timebase_wdt compatible ip cores.
It takes watchdog timer configuration from device tree and it needs that its
parent has defined the property "clock-frecuency".
It is compatible with watchdog timer kernel API, so user apps like watchdogd
may talk with it.

Signed-off-by: Alejandro Cabrera <aldaya@gmail.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>