Fix one remaining user of system_rev.  This patch is needed for
3430SDP ES2 to boot after 4a4ada55.

Bisected by Timo-Pekka Launonen's <ext-timo-pekka.launonen@nokia.com>
list robot.
Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Timo-Pekka Launonen <ext-timo-pekka.launonen@nokia.com>
Cc: Lauri Leukkunen <lauri.leukkunen@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>