MbedTLS: Add definition checks for TRNG

The compiler gave redefinition warnings if
MBEDTLS_ENTROPY_HARDWARE_ALT is defined in the the compile
definitions. A check is added to prevent this warning.

Author: boraozgen

connectivity/mbedtls/platform/inc/platform_mbed.h

@@ -40,8 +40,10 @@
 #endif  // (defined(FEATURE_PSA) && defined(MBEDTLS_ENTROPY_NV_SEED))
 
 #if DEVICE_TRNG
+#if !defined(MBEDTLS_ENTROPY_HARDWARE_ALT)
 #define MBEDTLS_ENTROPY_HARDWARE_ALT
 #endif
+#endif
 
 #if defined(MBEDTLS_CONFIG_HW_SUPPORT)
 #include "mbedtls_device.h"