intc/arm_gic: Refactor operations on the distributor

intc/arm_gic: Refactor operations on the distributor

In preparation for the virtualization extensions implementation,
refactor the name of the functions and macros that act on the GIC
distributor to make that fact explicit. It will be useful to
differentiate them from the ones that will act on the virtual
interfaces.

Signed-off-by: Luc Michel <luc.michel@greensocs.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Sai Pavan Boddu <sai.pavan.boddu@xilinx.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20180727095421.386-2-luc.michel@greensocs.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>