Docs: Select IPC model when building psa-arch-tests FF test suite

Building psa-arch-tests FF test suite without TF-M IPC model
enabled breaks the build.
Update build document to explicitly set TFM_PSA_API in the
command line to build psa-arch-test FF test suite.

Change-Id: Ifa8b1b318ba978bdb921f1e31262e5c520ec0c12
Signed-off-by: David Hu <david.hu@arm.com>
diff --git a/docs/getting_started/tfm_build_instruction.rst b/docs/getting_started/tfm_build_instruction.rst
index c4a495f..bfc36ce 100644
--- a/docs/getting_started/tfm_build_instruction.rst
+++ b/docs/getting_started/tfm_build_instruction.rst
@@ -353,7 +353,7 @@
 
     cd <base folder>
     cd trusted-firmware-m
-    cmake -S . -B cmake_build -DTFM_PLATFORM=mps2/an521 -DCMAKE_TOOLCHAIN_FILE=toolchain_GNUARM.cmake -DTEST_PSA_API=IPC
+    cmake -S . -B cmake_build -DTFM_PLATFORM=mps2/an521 -DCMAKE_TOOLCHAIN_FILE=toolchain_GNUARM.cmake -DTEST_PSA_API=IPC -DTFM_PSA_API=ON
     cmake --build cmake_build -- install
 
 Alternately using traditional cmake syntax
@@ -364,7 +364,7 @@
     cd trusted-firmware-m
     mkdir cmake_build
     cd cmake_build
-    cmake .. -DTFM_PLATFORM=mps2/an521 -DCMAKE_TOOLCHAIN_FILE=../toolchain_GNUARM.cmake -DTEST_PSA_API=IPC
+    cmake .. -DTFM_PLATFORM=mps2/an521 -DCMAKE_TOOLCHAIN_FILE=../toolchain_GNUARM.cmake -DTEST_PSA_API=IPC -DTFM_PSA_API=ON
     make install
 
 Location of build artifacts