hw/arm_gic: Move NCPU definition to arm_gic.c

hw/arm_gic: Move NCPU definition to arm_gic.c

Move the NCPU definition to arm_gic.c: the maximum number
of CPU interfaces is defined by the GIC architecture specification
to be 8, so we don't need to have this #define in each of the
sources files which currently includes arm_gic.c.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Evgeny Voevodin <e.voevodin@samsung.com>