soc/intel_adsp: Unify "firmware status register" window size

There is a diverged definition for SRAM_REG_FW_END, which exists to
prevent the Zephyr window initialization from changing values that
were already set by the ROM or bootloader (though this is incomplete,
as we're not ensuring the memory is actually the same space except by
convention; we also don't have any Zephyr-side visibility as to the
content of this struct).

That was silly; the only thing worse than one magic number is four
magic numbers in different files.  Write a formula that works for all
the platforms and put it in the C file where it's used.

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