tests: drivers: build_all: add build for wdt_xilinx_axi

There is currently not an in-tree Zephyr board which enables the
wdt_xilinx_axi driver as part of its standard configuration. (It can be
enabled fairly easily with the Digilent Arty board, but that requires
modifying the ARM-provided FPGA block design in Vivado and rebuilding
the FPGA image, so it cannot be used with the default FPGA image.)

Add a build_all target to provide build coverage for this driver.

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