hw/arm/virt: Add high MMIO PCI region, 512G in size

hw/arm/virt: Add high MMIO PCI region, 512G in size

This large region is necessary for some devices like ivshmem and video cards
32-bit kernels can be built without LPAE support. In this case such a kernel
will not be able to use PCI controller which has windows in high addresses.
In order to work around the problem, "highmem" option is introduced. It
defaults to on on, but can be manually set to off in order to be able to run
those old 32-bit guests.

Signed-off-by: Pavel Fedin <p.fedin@samsung.com>
Reviewed-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Shannon Zhao <shannon.zhao@linaro.org>
[PMM: Added missing ULL suffixes and a comment to the a15memmap[] entry]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>