OvmfPkg: Add the MIT license to License.txt.

OvmfPkg: Add the MIT license to License.txt.

The MIT license will be used for several source files that are necessary
for the Xen PV drivers. So this patch makes it explicit by adding the
license with a note about which directory will have source files under
this license.

Change in V3:
  New patch

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16256 6f19259b-4bc3-4df7-8a09-765794883524