ARM: highbank: select ARCH_HAS_HOLES_MEMORYMODEL

On Midway, the first 4G of memory starts at 0 and the rest of memory
(4GB+) starts at 0x2_00000000, so we need to enable pfn_valid checks
by selecting ARCH_HAS_HOLES_MEMORYMODEL.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>