soc/intel_adsp: Move boot stack to IMR memory

Put the initial stack in (slow) IMR memory instead of HP-SRAM.  Very
little happens on this stack anyway before Zephyr switches off to the
interrupt stack and/or main thread.  And this will allow us to start
running stackful C code much earlier in the boot process.

This puts the stack into the region of memory immediately below the
IMR image (because that edge of the image is easier to detect in
code).  On all current platforms, that's offset quite a bit into IMR
space and leaves plenty of room.  But we should have some kind
architectural way to reserve this space...

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