Integrate mbed OS RTC with mbed TLS

The integration is simply to define the macro MBEDTLS_HAVE_TIME_DATE
in the features/mbedtls/platform/inc/platform_mbed.h. The default
implementation of the mbedtls_time() function provided by mbed TLS is
sufficient to work with mbed OS because both use POSIX functions.

Author: Andres Amaya Garcia

features/mbedtls/platform/inc/platform_mbed.h

@@ -21,6 +21,10 @@
 #define MBEDTLS_ENTROPY_HARDWARE_ALT
 #endif
 
+#if defined(DEVICE_RTC)
+#define MBEDTLS_HAVE_TIME_DATE
+#endif
+
 #if defined(MBEDTLS_CONFIG_HW_SUPPORT)
 #include "mbedtls_device.h"
 #endif