hw/intc/arm_gic: Remove the definition of NUM_CPU

arm_gic.c retrieves CPU number using either NUM_CPU(s) or s->num_cpu.
Such mixed-uses make source code inconsistent. This patch removes
NUM_CPU(s), which was defined for MPCore tweak long ago, and instead
favors s->num_cpu. The source is more consistent after this small tweak.

Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Wei Huang <wei@redhat.com>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Message-id: 1446744293-32365-1-git-send-email-wei@redhat.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>