hw/arm: Add 'virt' platform

hw/arm: Add 'virt' platform

Add 'virt' platform support corresponding to arch/arm/mach-virt
in the Linux kernel tree. This has no platform-specific code but
can use any device whose kernel driver is is able to work purely
from a device tree node. We use this to instantiate a minimal
set of devices: a GIC and some virtio-mmio transports.

Signed-off-by: John Rigby <john.rigby@linaro.org>
Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
Message-id: 1385140638-10444-8-git-send-email-peter.maydell@linaro.org
[PMM:
 Significantly overhauled:
 * renamed user-facing machine to just "virt"
 * removed the A9 support (it can't work since the A9 has no
   generic timers)
 * added virtio-mmio transports instead of random set of 'soc' devices
   (though we retain a pl011 UART)
 * instead of updating io_base as we step through adding devices,
   define a memory map with an array (similar to vexpress)
 * similarly, define irqmap with an array
 * folded in some minor fixes from John's aarch64-support patch
 * rather than explicitly doing endian-swapping on FDT cells,
   use fdt APIs that let us just pass in host-endian values
   and let the fdt layer take care of the swapping
 * miscellaneous minor code cleanups and style fixes
]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>