ARM: 7397/1: l2x0: only apply workaround for erratum #753970 on PL310

ARM: 7397/1: l2x0: only apply workaround for erratum #753970 on PL310

commit f154fe9b806574437b47f08e924ad10c0e240b23 upstream.

The workaround for PL310 erratum #753970 can lead to deadlock on systems
with an L220 cache controller.

This patch makes the workaround effective only when the cache controller
is identified as a PL310 at probe time.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>