bionic-amd64-tf-m-build: Install ArmClang 6.20

It was found that 6.21 may trigger warning-as-error which really looks like
false positive (more info: https://linaro.atlassian.net/browse/TFC-540). That
warning doesn't happend with 6.18, so now going to try intermediate versions.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: Ie37fc95620fff51a49ee2ca5cd46d6696d264721
diff --git a/bionic-amd64-tf-m-build/tf-dependencies.install b/bionic-amd64-tf-m-build/tf-dependencies.install
index 8cb7d08..c2fb53b 100755
--- a/bionic-amd64-tf-m-build/tf-dependencies.install
+++ b/bionic-amd64-tf-m-build/tf-dependencies.install
@@ -49,6 +49,11 @@
 tar -xf /tmp/ARMCompiler6.18_standalone_linux-x86_64.tar.gz -C /tmp/armclang-6.18
 /tmp/armclang-6.18/install_x86_64.sh --no-interactive --i-agree-to-the-contained-eula -d ${ARMCLANG_6_18_DIR}
 
+# Install Arm Clang v6.20 toolchain to ARMCLANG_6_20_DIR (armclang)
+mkdir -p /tmp/armclang-6.20
+tar -xf /tmp/ARMCompiler6.20.1_standalone_linux-x86_64.tar.gz -C /tmp/armclang-6.20
+/tmp/armclang-6.20/install_x86_64.sh --no-interactive --i-agree-to-the-contained-eula -d ${ARMCLANG_6_20_DIR}
+
 # Install Arm Clang v6.21 toolchain to ARMCLANG_6_21_DIR (armclang)
 mkdir -p /tmp/armclang-6.21
 tar -xf /tmp/ARMCompiler6.21_standalone_linux-x86_64.tar.gz -C /tmp/armclang-6.21