soc/intel_adsp: Simplify boot ROM protocol

There was an attempt in the old code to express this as a formal
protocol with a proper field definitions, etc...  But in fact no such
protocol really exists.  This scheme is only used in one place to send
one specific message to code fixed in ROM on legacy devices that only
knows how to recognize this specific value.  And 2.5 and later
hardware are moving away from it anyway.

Just express it directly, and explain in comments.

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