boards: arm: xmc45_relax_kit: Add supported section in .yaml file

So that twister tests can be properly triggered on hardware. For example
using command:
$ scripts/twister --device-testing --device-serial /dev/ttyUSB0 \
--device-serial-baud 115200 -p xmc45_relax_kit --fixture gpio_loopback \
-T tests/drivers/uart/uart_async_api/

Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>