ARM: highbank: Add smc calls to enable/disable the L2

Linux runs in non-secure mode on highbank, so we need secure monitor calls
to enable and disable the PL310. Rather than invent new smc calls, the same
calling convention used by OMAP is used here.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Olof Johansson <olof@lixom.net>