zephyr: mec15xxevb_assy6853: broken UART console output

Describe the bug mec15xxevb_assy6853 rev.A broken (binary) output at its UART console. Bisect points to commit c7e0d727 introduced with #52303

To Reproduce Steps to reproduce the behavior:

1. west build -p always -b mec15xxevb_assy6853 samples/hello_world
2. flash the board as usual
3. check console output at another shell with e.g. picocom -b 115200 /dev/ttyUSB1

Expected behavior correct console output

Impact all our test cases fail to get expected console output

Logs and console output corrupted output (with commit c7e0d727) correct output (for previous commit)

Environment (please complete the following information):

• OS: Fedora Linux kernel 5.10.12-200.fc33.x86_64
• Toolchain zephyr-sdk-0.15.0
• Commit c7e0d727

Additional context n/a