ArmPkg: import Berkeley Softfloat library as git submodule

In preparation of bringing ArmSoftFloatLib up to date in order
to provide some missing routines, import the Berkely SoftFloat
library into the tree as a git submodule.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1845
Acked-by: Jian J Wang <jian.j.wang@intel.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

.gitmodules

@@ -1,3 +1,6 @@
 [submodule "CryptoPkg/Library/OpensslLib/openssl"]
 	path = CryptoPkg/Library/OpensslLib/openssl
 	url = https://github.com/openssl/openssl
+[submodule "SoftFloat"]
+	path = ArmPkg/Library/ArmSoftFloatLib/berkeley-softfloat-3
+	url = https://github.com/ucb-bar/berkeley-softfloat-3.git