tfm: remove @return doc for void functions

For functions returning nothing, there is no need to document
with @return, as Doxgen complains about "documented empty
return type of ...".

Signed-off-by: Daniel Leung <daniel.leung@intel.com>

modules/trusted-firmware-m/src/reboot.c

@@ -22,8 +22,6 @@
  * the weak implementation of sys_arch_reboot() in scb.c.
  *
  * \pre The implementation requires the TFM_PARTITION_PLATFORM be defined.
- *
- * @return N/A
  */
 
 #if defined(CONFIG_TFM_PARTITION_PLATFORM)

samples/tfm_integration/tfm_ipc/src/main.c

@@ -37,8 +37,6 @@ static void tfm_ipc_test_01(void)
 
 /**
  * Retrieve the minor version of a RoT Service.
- *
- * @return N/A
  */
 static void tfm_ipc_test_02(void)
 {
@@ -57,8 +55,6 @@ static void tfm_ipc_test_02(void)
 
 /**
  * Connect to a RoT Service by its SID.
- *
- * @return N/A
  */
 static void tfm_ipc_test_03(void)
 {