exec/memory: Add symbol for the min value of memory listener priority

Add MEMORY_LISTNER_PRIORITY_MIN for the symbolic value for the min value of
the memory listener instead of the hard-coded magic value 0.  Add explicit
initialization.

No functional change intended.

Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <29f88477fe82eb774bcfcae7f65ea21995f865f2.1687279702.git.isaku.yamahata@intel.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>