Rename hash max sizes for consistency

Use "hash" throughout the library, not "md" as in Mbed TLS.
diff --git a/include/psa/crypto_struct.h b/include/psa/crypto_struct.h
index 4b0f979..0dbd86c 100644
--- a/include/psa/crypto_struct.h
+++ b/include/psa/crypto_struct.h
@@ -46,11 +46,11 @@
 #include "mbedtls/sha512.h"
 
 #if defined(MBEDTLS_SHA512_C)
-#define PSA_CRYPTO_MD_MAX_SIZE 64
-#define PSA_CRYPTO_MD_BLOCK_SIZE 128
+#define PSA_HASH_MAX_SIZE 64
+#define PSA_HMAC_MAX_HASH_BLOCK_SIZE 128
 #else
-#define PSA_CRYPTO_MD_MAX_SIZE 32
-#define PSA_CRYPTO_MD_BLOCK_SIZE 64
+#define PSA_HASH_MAX_SIZE 32
+#define PSA_HMAC_MAX_HASH_BLOCK_SIZE 64
 #endif
 
 struct psa_hash_operation_s
@@ -89,7 +89,7 @@
         /** The hash context. */
         struct psa_hash_operation_s hash_ctx;
         /** The HMAC part of the context. */
-        uint8_t opad[PSA_CRYPTO_MD_BLOCK_SIZE];
+        uint8_t opad[PSA_HMAC_MAX_HASH_BLOCK_SIZE];
 } psa_hmac_internal_data;