ARM: 7398/1: l2x0: only write to debug registers on PL310

ARM: 7398/1: l2x0: only write to debug registers on PL310

commit ab4d536890853ab6675ede65db40e2c0980cb0ea upstream.

PL310 errata #588369 and #727915 require writes to the debug registers
of the cache controller to work around known problems. Writing these
registers on L220 may cause deadlock, so ensure that we only perform
this operation when we identify 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>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>