FatPkg: Update License.txt to have the full license text

FatPkg: Update License.txt to have the full license text

A script was used to convert the license text for the entire FatPkg.
It used the shorter BSD text that we use in most files:

  This program and the accompanying materials are licensed and made
  available under the terms and conditions of the BSD License which
  accompanies this distribution. The full text of the license may be
  found at http://opensource.org/licenses/bsd-license.php

The script also used the short version in FatPkg/License.txt. In each
package's License.txt, we normally include the full BSD license text.
This change copies the longer version of License.txt from MdePkg.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>