ARM: 8659/1: l2c: allow CA9 optimizations to be disabled

If a PL310 is added to a system, but the sideband signals are not
connected, some Cortex A9 optimizations cannot be used. In particular,
enabling Full Line of Zeros in the CA9 without sidebands connected will
crash the system since the CA9 will expect the L2C to perform operations,
yet the L2C never gets the commands. Early BRESP also does not work
without sideband signals.

Signed-off-by: Chris Brandt <chris.brandt@renesas.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>