tests: drivers: uart: uart_mix_fifo_poll: hags at nrf54h20 and nrf54l15

[nordic-piks]

Describe the bug
Tests:

• tests/drivers/uart/uart_mix_fifo_poll/drivers.uart.uart_mix_poll_fifo
• tests/drivers/uart/uart_mix_fifo_poll/drivers.uart.uart_mix_poll_fifo_with_ppi
• tests/drivers/uart/uart_mix_fifo_poll/drivers.uart.uart_mix_poll_async_api_with_ppi
• tests/drivers/uart/uart_mix_fifo_poll/drivers.uart.legacy.uart_mix_poll_fifo

Observed for

• nrf54l15pdk/nrf54l15/cpuapp
• nrf54h20dk/nrf54h20/cpuapp
• nrf54h20dk/nrf54h20/cpurad

To Reproduce
Steps to reproduce the behavior:

1. have nrf54l15pdk/nrf54l15/cpuapp connected
2. go to your zephyr dir
3. call ./scripts/twister -T tests/drivers/uart/uart_mix_fifo_poll -p nrf54l15pdk/nrf54l15/cpuapp --device-testing --device-serial /dev/ttyACM0 -v --inline-logs
4. See console output with error

Expected behavior
Valid console output

Impact
Not clear

Logs and console output

*** Booting Zephyr OS build v3.6.0-4231-g7046c9030e70 ***
Running TESTSUITE uart_mix_fifo_poll
===================================================================
START - test_mixed_uart_access

Test hangs

Environment (please complete the following information):

• OS: Ubuntu 20.04.1 LTS
• Toolchain Zephyr SDK 0.16.5
• Commit SHA or Version used: v3.6.0-4231-g7046c9030e70

[nordic-piks]

@nordic-krch Found after #72548

[nordic-krch]

It hangs after d599e2b @aescolar @adamkondraciuk any ideas?
Test is extensively using timers and there are 2 interrupt levels (GRTC runs from lower priority timer than UARTE, TIMER).