fix(st): update comment on encryption key

On STM32MP2, the encryption key is 32 bytes, the key duplication
(done for 16 bytes OTP) is not done. Update the comment to precise that.

Change-Id: I6fc4d652fdd462808918e85f6e5bd0d68d10d436
Yann Gautier <yann.gautier@foss.st.com>

plat/st/common/stm32mp_crypto_lib.c

@@ -491,7 +491,7 @@ static int derive_key(uint8_t *key, size_t *key_len, size_t len,
 	/*
 	 * Not a real derivation yet
 	 *
-	 * But we expect a 32 bytes key, and OTP is only 16 bytes
+	 * We expect a 32 bytes key, if OTP is only 16 bytes
 	 *   => duplicate.
 	 */
 	for (i = 0U, j = len; j < 32U;