Platform: Add system reset hal function in SPM

Add tfm_spm_hal_system_reset() function which is used by SPM to reset
the system.
All platforms have the same implementation except for Musca-B1.

Change-Id: I1fe34d9026b9ec35bf981ea9fc6b5a92d391ed58
Signed-off-by: Edison Ai <edison.ai@arm.com>
diff --git a/platform/ext/common/tfm_platform.c b/platform/ext/common/tfm_platform.c
index 48e56f1..eabe3a4 100644
--- a/platform/ext/common/tfm_platform.c
+++ b/platform/ext/common/tfm_platform.c
@@ -26,3 +26,8 @@
 
     return TFM_PLAT_ERR_SUCCESS;
 }
+
+__WEAK void tfm_spm_hal_system_reset(void)
+{
+    NVIC_SystemReset();
+}