tests: kernel: usage: skip for mr_canhubk3 board

The mr_canhubk3 board enables by default an off-chip watchdog that must
be serviced to avoid triggering a reset and cannot be disabled on a
per-test basis.

test_all_stats_usage assumes the CPU was never idle before the test
starts but this is not the case for mr_canhubk3 because the off-chip
watchdog driver has a thread kicked off during device init that will
conflict with the expected usage stats on this test. So skip this test
for this board.

Signed-off-by: Manuel Argüelles <manuel.arguelles@nxp.com>