RealView: Allow PHYS_OFFSET at 0x70000000

RealView boards like PB11MPCore have 512MB of RAM available contiguously
at 0x70000000. Half of the memory is mirrored at 0x00000000 for
backwards compatibility. This patch adds the
CONFIG_REALVIEW_HIGH_PHYS_OFFSET option option to change the physical
base address so that the full amount of RAM is available to Linux. Note
that the EB board has 256MB of RAM also mirrored at 0x70000000, the only
board without this feature being PB1176.

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