Replace MBEDTLS_MD_CAN_SHA256 in md.h, ssl.h, and entropy.h

Signed-off-by: Elena Uziunaite <elena.uziunaite@arm.com>
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 3781b51..8f019c1 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -1153,7 +1153,7 @@
 #if defined(MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED) &&        \
     !defined(MBEDTLS_SSL_KEEP_PEER_CERTIFICATE)
 #define MBEDTLS_SSL_PEER_CERT_DIGEST_MAX_LEN  48
-#if defined(MBEDTLS_MD_CAN_SHA256)
+#if defined(PSA_WANT_ALG_SHA_256)
 #define MBEDTLS_SSL_PEER_CERT_DIGEST_DFL_TYPE MBEDTLS_MD_SHA256
 #define MBEDTLS_SSL_PEER_CERT_DIGEST_DFL_LEN  32
 #elif defined(PSA_WANT_ALG_SHA_384)