Build: Rename TFM_PLATFORM

Arm platforms are moved into arm folder, so the build command
TFM_PLATFORM should do the corresponding changes.

Signed-off-by: Summer Qin <summer.qin@arm.com>
Change-Id: Iba23e4f71d5c5865f358c2b7ce612ba603eff4e7
diff --git a/build-docs.sh b/build-docs.sh
index 1a87450..e4fb66d 100755
--- a/build-docs.sh
+++ b/build-docs.sh
@@ -1,6 +1,6 @@
 #!/bin/bash
 #-------------------------------------------------------------------------------
-# Copyright (c) 2020, Arm Limited and Contributors. All rights reserved.
+# Copyright (c) 2020-2021, Arm Limited and Contributors. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
@@ -19,7 +19,7 @@
 mkdir ../trusted-firmware-m/build
 cd ../trusted-firmware-m/build
 
-cmake -S .. -B . -DTFM_PLATFORM=mps2/an521 \
+cmake -S .. -B . -DTFM_PLATFORM=arm/mps2/an521 \
                  -DTFM_TOOLCHAIN_FILE=../toolchain_GNUARM.cmake \
                  -DMBEDCRYPTO_PATH=../../mbedtls \
                  -DTFM_TEST_REPO_PATH=../../tf-m-tests \