hal_ethos_u: decouple ETHOS_U_LOG_LEVEL from STDOUT_CONSOLE

Previously the ETHOS_U_LOG_LEVEL choice and int symbol only existed
when STDOUT_CONSOLE was enabled. This caused build and compile issues
if logging was disabled, since CONFIG_ETHOS_U_LOG_LEVEL would be
undefined and C code using LOG_MODULE_REGISTER() would fail.

- Replace dependency on STDOUT_CONSOLE with LOG.
- Ensure ETHOS_U_LOG_LEVEL is always defined, even when LOG is off.
  In that case default to 0 (NONE).

This guarantees a valid CONFIG_ETHOS_U_LOG_LEVEL is present in all
configurations and allows clean builds regardless of console settings.

Signed-off-by: Johan Alfvén <johan.alfven@arm.com>