Favor stdint.h types in internal types

Use uint8_t for PSA buffers. Keep unsigned char for generic libc
buffers and for mbedtls buffers.

include/psa/crypto_struct.h

@@ -210,7 +210,7 @@ typedef struct psa_tls12_prf_key_derivation_s
      * hence we must store it for the lifetime of the operation.
      * This is different from HKDF, where the key is only used
      * in the extraction phase, but not during expansion. */
-    unsigned char *key;
+    uint8_t *key;
     size_t key_len;
 
     /* `A(i) + seed` in the notation of RFC 5246, Sect. 5 */