soc: intel_adsp: New IDC driver

The original interface for the intra-DSP communication hardware on
these devices was buried inside a Zephyr IPM implementation.
Unfortunately IPM is a two-endpoint point-to-point communication
layer, it can't represent the idea of devices with more than 2 cores.
And our usage (to push a no-argument/no-response scheduler IPI) was
sort of an abuse of that metaphor anyway.

Add a new IDC interface at the SOC layer, borrowing the C struct
convention already used for the DSP shim registers.

Augment with extensive documentation, extracted via a ton of
experimentation on cAVS 2.5 hardware.

Note that this leaves the previous driver in place for the cavs_v15
and intel_s1000 devices.  In principle they should use it too (the
hardware registers are identical), but this hasn't been validated yet.

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