hw/arm_gic: Convert ARM GIC classes to use init/realize

Convert the ARM GIC classes to use init/realize rather than
SysBusDevice::init. (We have to do them all in one patch to
avoid unconverted subclasses calling a nonexistent SysBusDevice
init function in the base class and crashing.)

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