Simplify MBEDTLS_TIMING_C to use mbedtls_ms_time()

Don't ship two slightly different wheels.

This reduces our platform adherence by using only `clock_gettime()` in the
library and not `gettimeofday()` as well.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

include/mbedtls/timing.h

@@ -23,11 +23,13 @@ extern "C" {
 // Regular implementation
 //
 
+#include <mbedtls/platform_time.h>
+
 /**
  * \brief          timer structure
  */
 struct mbedtls_timing_hr_time {
-    uint64_t MBEDTLS_PRIVATE(opaque)[4];
+    mbedtls_ms_time_t MBEDTLS_PRIVATE(ms);
 };
 
 /**