power_domain_intel_adsp.c: revert recent INIT_PRIORITY change

This is a partial revert of one-line from commit 06cfbd41 ("drivers:
power_domain: Introduce a gpio monitor driver") which not just
introduced a new driver (no problem with that) but also changed the
initialization priority of another, unrelated and existing power domain
driver without even trying to compile it:

https://github.com/zephyrproject-rtos/zephyr/pull/61166#issuecomment-1780959157

```
west config manifest.project-filter -- +sof
west update

west build -b intel_adsp_ace20_lnl modules/audio/sof/app/

ERROR: /soc/ssp@28100 POST_KERNEL 43 < /soc/dfpmccu@71b00/io0_domain  51
ERROR: /soc/ssp@29100 POST_KERNEL 44 < /soc/dfpmccu@71b00/io0_domain  51
ERROR: /soc/ssp@2a100 POST_KERNEL 45 < /soc/dfpmccu@71b00/io0_domain  51
ERROR: /soc/ssp@2b100 POST_KERNEL 46 < /soc/dfpmccu@71b00/io0_domain  51
ERROR: /soc/ssp@2c100 POST_KERNEL 47 < /soc/dfpmccu@71b00/io0_domain  51
ERROR: /soc/ssp@2d100 POST_KERNEL 48 < /soc/dfpmccu@71b00/io0_domain  51
```

Also note a reviewer (@ceolin) expressed concerns about this unrelated
change but it was ignored:
https://github.com/zephyrproject-rtos/zephyr/pull/61166#discussion_r1357908984

Using `CONFIG_KERNEL_INIT_PRIORITY_DEFAULT` here may be "bad" for some
reason(s) and maybe it should be changed in the future, but it's nothing
compared to breaking _compilation_ of code that has been validated for
months and been released in production
(https://github.com/thesofproject/sof-bin/releases/tag/v2023.09

)

So the very urgent thing is to very quickly revert to the previous state
to unblock development. Then we can discuss what is the better thing to
do here.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>