Tianocore: Support FDT library.

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=4392
Add FDT support in EDK2 by submodule 3rd party libfdt
(https://github.com/devicetree-org/pylibfdt/tree/main/libfdt)

Add submodule libfdt and update ReadMe for the license.

Cc: Andrew Fish <afish@apple.com>
Cc: Leif Lindholm <quic_llindhol@quicinc.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Signed-off-by: Benny Lin <benny.lin@intel.com>
Reviewed-by: Leif Lindholm <quic_llindhol@quicinc.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>

.gitmodules

@@ -26,3 +26,6 @@
 [submodule "UnitTestFrameworkPkg/Library/SubhookLib/subhook"]
 	path = UnitTestFrameworkPkg/Library/SubhookLib/subhook
 	url = https://github.com/Zeex/subhook.git
+[submodule "MdePkg/Library/BaseFdtLib/libfdt"]
+	path = MdePkg/Library/BaseFdtLib/libfdt
+	url = https://github.com/devicetree-org/pylibfdt.git

ReadMe.rst

@@ -96,6 +96,7 @@ that are covered by additional licenses.
 -  `UnitTestFrameworkPkg/Library/GoogleTestLib/googletest <https://github.com/google/googletest/blob/86add13493e5c881d7e4ba77fb91c1f57752b3a4/LICENSE>`__
 -  `UnitTestFrameworkPkg/Library/SubhookLib/subhook <https://github.com/Zeex/subhook/blob/83d4e1ebef3588fae48b69a7352cc21801cb70bc/LICENSE.txt>`__
 -  `RedfishPkg/Library/JsonLib/jansson <https://github.com/akheron/jansson/blob/2882ead5bb90cf12a01b07b2c2361e24960fae02/LICENSE>`__
+-  `MdePkg/Library/BaseFdtLib/libfdt <https://github.com/devicetree-org/pylibfdt/blob/f39368a217496d32c4091a2dba4045b60649e3a5/BSD-2-Clause>`__
 
 The EDK II Project is composed of packages. The maintainers for each package
 are listed in `Maintainers.txt <Maintainers.txt>`__.