drivers: watchdog: add Xilinx AXI Timebase WDT driver

Add a driver for the Xilinx AXI Timebase WDT logic core. This can be
instantiated on various Xilinx FPGA-based platforms such as the
Digilent Arty, although it is not part of the default image used with
the Zephyr board configuration.

The driver can also optionally implement the HWINFO API to allow
determining whether the last system reset was initiated by the WDT.
Since this is a standalone IP core which could be used a variety of
configurations, this support is optional in case the system/SoC it is
used with already implements this support.

Signed-off-by: Robert Hancock <robert.hancock@calian.com>