Replace mbedtls_platform_enforce_volatile_reads

Replace function mbedtls_platform_enforce_volatile_reads() with
mbedtls_platform_random_delay().
diff --git a/tinycrypt/ecc_dsa.c b/tinycrypt/ecc_dsa.c
index ca071f8..c19c73e 100644
--- a/tinycrypt/ecc_dsa.c
+++ b/tinycrypt/ecc_dsa.c
@@ -299,7 +299,7 @@
 	/* Accept only if v == r. */
 	diff = uECC_vli_equal(rx, r);
 	if (diff == 0) {
-		mbedtls_platform_enforce_volatile_reads();
+	    mbedtls_platform_random_delay( 50 );
 		if (diff == 0) {
 			return UECC_SUCCESS;
 		}