hw/arm/virt: Add gic-version option to virt machine

Add gic_version to VirtMachineState, set it to value of the option
and pass it around where necessary. Instantiate devices and fdt
nodes according to the choice.

max_cpus for virt machine increased to 123 (calculated from redistributor
space available in the memory map). GICv2 compatibility check happens
inside arm_gic_common_realize().

ITS region is added to the memory map too, however currently it not used,
just reserved.

Signed-off-by: Pavel Fedin <p.fedin@samsung.com>
Tested-by: Ashok kumar <ashoks@broadcom.com>
[PMM: Added missing cpu_to_le* calls, thanks to Shannon Zhao]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
