PEM: always use MD light

Note: PEM_PARSE already auto-enables MD_LIGHT in build_info.h

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>

include/mbedtls/mbedtls_config.h

@@ -2775,6 +2775,10 @@
  *          library/x509_csr.c
  *
  * Requires: MBEDTLS_BASE64_C
+ *           optionally MBEDTLS_MD5_C, or PSA Crypto with MD5 (see below)
+ *
+ * \warning When parsing password-protected files, if MD5 is provided only by
+ * a PSA driver, you must call psa_crypto_init() before the first file.
  *
  * This modules adds support for decoding / parsing PEM files.
  */