meta-ampere: Initial AmpereOne(TM) Mt. Mitchell's non-DC-SCM BMC support

Add new Mt.Mitchell platform with basic features so that it can be
compiled and works.

Tested:
1. Compile for Mt.Mitchell successful.
2. Flash and check for the firmware can boot on the Mt.Mitchell
non-DC-SCM system without issue

Signed-off-by: Chanh Nguyen <chanh@os.amperecomputing.com>
Change-Id: I8dc9db9029b27c0c6325c31d368cdbb614a3c8b6