Add milliseconds time function

We provide windows and posix implementation for it.
With MBEDTLS_PLATFORM_MS_TIME_ALT, user can provide
their own implementation.

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/include/mbedtls/mbedtls_config.h b/include/mbedtls/mbedtls_config.h
index 34fdfcc..cfde4b7 100644
--- a/include/mbedtls/mbedtls_config.h
+++ b/include/mbedtls/mbedtls_config.h
@@ -234,6 +234,7 @@
 //#define MBEDTLS_PLATFORM_VSNPRINTF_ALT
 //#define MBEDTLS_PLATFORM_NV_SEED_ALT
 //#define MBEDTLS_PLATFORM_SETUP_TEARDOWN_ALT
+//#define MBEDTLS_PLATFORM_MS_TIME_ALT
 
 /**
  * \def MBEDTLS_DEPRECATED_WARNING