Uniformly spell frequency unit symbol as Hz

Change-Id: I1eb8d5bd79322ff3654a6ad66278a57d46a818c1
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: http://review.coreboot.org/6384
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>