soc: intel_adsp: General soc_mp.c cleanup

Misc cleanup, no non-trivial logic changes.

Swap in new ("rsr <reg>, REGISTER_NAME") syntax for Xtensa SR's in
place of inconsistent usage of the older one ("rsr.REGISTER_NAME
<reg>").

Remove the legacy handling of !KERNEL_COHERENCE cases for allocating
the cpu start record.  That has long been a requirement of
multiprocessor code on this platform.

Remove the synchronous testing of the "alive" flag in
arch_start_cpu().  Nothign about that API is intended to be
synchronous, and in fact the Zephyr SMP layer is already doing the
same trick.

Remove some vestigial dead code at the end of z_mp_entry().  It was
apparently intended to handle the case where a CPU function returned,
but that's not legal anyway.  And it was only enabled in the case
where there was only one CPU anyway, which was an impossible situation
(you can't evercall arch_start_cpu() successfully on a system with
only one core, for obvious reasons -- the only core is already
running!).  Replace with an assertion.

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