lib: rsa: Leverage existing data buffer instead of systematic copy

Prior to introduction of modifications in rsassa_pss functions
related to padding verification, doing a pass to reduce memory
consumption of function by replacing memory copies of parts of
const buffer by pointers to the original buffer (masked_db and
h are subparts of msg buffer which is declared const, salt is a
subpart of db which is a working buffer, unmodified after being
filled). New pointers scope is limited to the function where
they are declared (not returned to caller by any mean), zeroing
risk of memory fault related to the change.

Signed-off-by: SESA644425 <gioja.hermann@non.se.com>
Reviewed-by: Simon Glass <sjg@chromium.org>