MdeModulePkg/BrotliCustomDecompressLib: Make brotli a submodule

Use submodule way to access brotli in MdeModulePkg based on
brotli version 666c3280cc11dc433c303d79a83d4ffbdd12cc8d.
The newly added BrotliDecUefiSupport.h/.c are used by directory
'brotli'.
REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2559

Cc: Liming Gao <liming.gao@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Hao A Wu <hao.a.wu@intel.com>
Signed-off-by: Shenglei Zhang <shenglei.zhang@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

MdeModulePkg/Library/BrotliCustomDecompressLib/stdint.h

@@ -0,0 +1,9 @@
+/** @file
+  Include file to support building the third-party brotli.
+
+Copyright (c) 2020, Intel Corporation. All rights reserved.<BR>
+SPDX-License-Identifier: BSD-2-Clause-Patent
+
+**/
+
+#include <BrotliDecUefiSupport.h>