Update SimpleFileSystem.h to add the asynchronous File IO API definition.

Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Feng Tian <feng.tian@intel.com>

git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/MdePkg@14818 6f19259b-4bc3-4df7-8a09-765794883524