tests: drivers: udc: Disable test on nRF54LM20dk

After adding 'usbd' to the supported peripherals on nrf54lm20
Twister started executing drivers.usb.udc test.
However, test fails to build due to 'dwc2_vendor_quirks_0' undeclared.

Temporarily disable this test on nrf54lm20dk target.

Signed-off-by: Sebastian Głąb <sebastian.glab@nordicsemi.no>

Author: nordic-segl

tests/drivers/udc/testcase.yaml

@@ -13,6 +13,7 @@ tests:
       - rpi_pico
     platform_exclude:
       - nrf54h20dk/nrf54h20/cpuapp
+      - nrf54lm20dk/nrf54lm20a/cpuapp
   drivers.usb.udc.build_only:
     build_only: true
     # Platform excluded above, only to build, with justification.