arm/boot: Allow boards to modify the FDT blob

arm/boot: Allow boards to modify the FDT blob

Add a callback hook in arm_boot_info to allow board models to
modify the device tree blob if they need to. (The major expected
use case is to add virtio-mmio nodes for virtio-mmio transports
that exist in QEMU but not in the hardware.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1373977512-28932-7-git-send-email-peter.maydell@linaro.org