dev, arm: Add a platform with support for both aarch32 and aarch64

dev, arm: Add a platform with support for both aarch32 and aarch64

Add a platform with support for both aarch32 and aarch64. This
platform implements a subset of the devices in a real Versatile
Express and extends it with some gem5-specific functionality. It is in
many ways similar to the old VExpress_EMM64 platform, but supports the
following new features:

  * Automatic PCI interrupt assignment
  * PCI interrupts allocated in a contiguous range.
  * Automatic boot loader selection (32-bit / 64-bit)
  * Cleaner memory map where gem5-specific devices live in CS5 which
    isn't used by current Versatile Express platforms.
  * No fake devices. Devices that were previously faked will be
    removed from the device tree instead.
  * Support for 510 GiB contiguous memory