RPC: Add the dedicated eRPC config file in test repo

Change-Id: I98236b2af8fd850c705c182ba0768feab94b13b1
Signed-off-by: Kevin Peng <kevin.peng@arm.com>
diff --git a/docs/tfm_erpc_test_framework.rst b/docs/tfm_erpc_test_framework.rst
index 5fa33ee..146e12e 100644
--- a/docs/tfm_erpc_test_framework.rst
+++ b/docs/tfm_erpc_test_framework.rst
@@ -140,13 +140,18 @@
 When the eRPC test framework is enabled, the ``CONFIG_ENABLE_NS_UART_TX_RX_CONTROL`` will be enabled
 automatically.
 
-Secondly, platforms need to specify the UART port and driver for eRPC transportation config via
-``target_cfg.h``.
+Secondly, platforms need to create their folders under the ``erpc/platform`` and then create the
+``config_erpc_target.h`` to specify the UART port drivers for eRPC transportation.
 
 .. code-block::
 
   #define ERPC_UART           Driver_USART0
 
+.. note::
+
+  The folder structure in ``erpc/platform`` must be the same as the ``platform/ext/target`` of TF-M
+  repo.
+
 It's recommended to use a different UART to the stdio UART.
 If the same UART is used for both, then the TF-M logs (both SPM and Secure Partitions) must be
 disabled.