soc/intel_adsp: Minimize bootloader linker script

The memory layout for the boot loader was needlessly complicated, with
separate fixed regions defined at fixed addresses, all in a file that
needs to be contiguous and DMA'd from the host in a single block.  The
end result was a lot of magic numbers and wasted space.

Clean things up so that it links in a single region expressed (for the
benefit of rimage, I think) as a single program header in the ELF
file.

This is in preparation for further changes to unify the bootloader
stage with the main Zephyr image in a single link.

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