UART input does not work on mps2_an{385,521}

[stephanosio]

Describe the bug
UART input does not work on mps2_an385 and mps2_an521 boards (in QEMU).

For example, the tests and samples that rely on the UART input functionality such as drivers/uart/uart_basic_api and shell are unable to accept any user input.

To Reproduce
In QEMU, run any tests and samples that require user input through the UART interface such as:

• drivers/uart/uart_basic_api
• samples/subsys/shell/shell_module

Expected behavior
UART user input is echo-ed on the input prompt.

Impact
Board usability is limited

Environment (please complete the following information):

• OS: Ubuntu 18.04
• Toolchain: Zephyr SDK 0.11.2
• Commit SHA: 855b6c1

Additional context
Broken UART driver suspected

Also reproducible in H/W (see #25601 (comment)).

[carlescufi]

Removing the priority, we will go through it later today.

[galak]

@microbuilder can you try one of these tests/samples on a real MPS2 to see if this shows up on real HW as well.

[microbuilder]

Repeatable on actual HW as well (UART input also doesn't work, output from AN521 -> USB Serial does work fine, as is the case with QEMU).

[github-actions]

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