RealView: Add sparsemem support for the RealView PBX platform

The RealView PBX board has two 512MB blocks of memory - one at
0x70000000 (with 256MB mirror at 0) and another at 0x20000000. Only the
block at 0x70000000 (or the mirror at 0) may be used for DMA (e.g.
framebuffer). This patch adds the sparsemem definitions to allow the use
of all the memory split as follows:

  256MB @ 0x00000000 (ZONE_DMA)
  512MB @ 0x20000000 (ZONE_NORMAL)
  256MB @ 0x80000000 (ZONE_NORMAL)

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>