[nrf fromtree] tests: drivers: uart: uart_pm: Disable PM_DEVICE_SYSTEM_MANAGED

When PM_DEVICE_SYSTEM_MANAGED is enabled (and that's the default) all
devices are suspended when device go to idle state (through pm
subsystem and not through just cpu_idle). In this test we are manually
controlling PM of the DUT so we don't want that.

Additionally, suspend action is taking a semaphore and it is illegal
in idle thread (where system managment operates) which causes assert
in kernel scheduler.

Removed for nrf54h20 as it applies only there.

Signed-off-by: Krzysztof Chruściński <[email protected]>

Signed-off-by: Sebastian Głąb <[email protected]>
(cherry picked from commit cbe0abf)

Author: nordic-segl

tests/drivers/uart/uart_pm/boards/nrf54h20dk_nrf54h20_cpuapp.conf

@@ -0,0 +1 @@
+CONFIG_PM_DEVICE_SYSTEM_MANAGED=n