soc/intel_adsp: Move window setup into bootloader

The bootloader had always been doing setup for window 0 (the firmware
status word reported to the loader).  This then got needlessly
repeated in the main OS startup.  Drop that, and move window 3 setup
(the trace/printk output buffer) there too for symmetry.

This means that the clear and cache flush of the buffer can be handled
in only one place too, for some runtime win (before it was being
cleared/flushed once at SRAM initialization time and then again at
window setup).

And as that was the only task remaining in the "adsp.c" file, we can
just remove it entirely.

One nice side effect is that this pushes up the point where we can get
a successful printk() out all the way back into the later stages of
the bootloader.

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