soc/intel_adsp: Move initialization code to IMR

Now that we have access to IMR memory for non-bootloader tasks, let's
pick the low hanging fruit.  SOC code that is only used at
initialization time (or things like core halt/restart which happen
only in non-realtime contexts) are now flagged __imr.

This is good for 808 bytes of code moved out of the main Zephyr image
on cavs_v25.

In the medium term, it would be good to define a system define for
this purpose (a-la Linux __init/__initdata) and start moving core
Zephyr init code too.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>