ARM: highbank: avoid L2 cache smc calls when PL310 is not present

While Midway firmware handles L2 smc calls as nops, the custom smc calls
present a problem when running virtualized Midway guest. They aren't
needed so just avoid calling them.

In the process, cleanup the L2X0 ifdefs and use IS_ENABLED instead.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>