ARM: 7608/1: l2x0: Only set .set_debug on PL310 r3p0 and earlier

PL310 errata work-arounds using .set_debug function are only needed on
r3p0 and earlier, so check the rev and only set .set_debug on older revs.

Avoiding debug register accesses fixes aborts on non-secure platforms
like highbank. It is assumed that non-secure platforms needing these
work-arounds have already implemented .set_debug with secure monitor
calls.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>