Merge pull request #4162 from adustm/STM_sha256_f756zg

NUCLEO_F756ZG/mbedtls: Add hw acceleration for SHA256

Author: theotherjimmy

features/mbedtls/targets/TARGET_STM/TARGET_STM32F7/TARGET_NUCLEO_F756ZG/mbedtls_device.h

@@ -24,5 +24,6 @@
 #define MBEDTLS_SHA1_ALT
 
 
+#define MBEDTLS_SHA256_ALT
 
 #endif /* MBEDTLS_DEVICE_H */