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/scripts/config.py b/scripts/config.py
index a53c470..06fc3fa 100755
--- a/scripts/config.py
+++ b/scripts/config.py
@@ -229,7 +229,7 @@
     Exclude alternative implementations of library functions since they require
     an implementation of the relevant functions and an xxx_alt.h header.
     """
-    if name == 'MBEDTLS_PLATFORM_SETUP_TEARDOWN_ALT':
+    if name in ('MBEDTLS_PLATFORM_SETUP_TEARDOWN_ALT', 'MBEDTLS_PLATFORM_MS_TIME_ALT'):
         # Similar to non-platform xxx_ALT, requires platform_alt.h
         return False
     return name.startswith('MBEDTLS_PLATFORM_')