ARM: rpc: make default fiq handler run-time installed

Only rpc uses disable_fiq macro. Change it to a run-time installed
default FIQ handler. The handler is installed before FIQ is enabled
so the behavior should be unchanged.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Nicolas Pitre <nico@linaro.org>