arm_gic: Rename gic_state to GICState

arm_gic: Rename gic_state to GICState

Rename the gic_state struct to match QEMU's coding style conventions
for structure names, since the impending KVM-for-ARM patches will
create another subclass of it. This patch was created using:
  sed -i 's/gic_state/GICState/g' hw/arm_gic.c hw/arm_gic_common.c \
    hw/arm_gic_internal.h hw/armv7m_nvic.c

Acked-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>