ARM: realview: fix sparsemem build

Commit b713aa0b "ARM: fix asm/memory.h build error" broke some
configurations on mach-realview with sparsemem enabled, which
is missing a definition of PHYS_OFFSET:

arch/arm/include/asm/memory.h:268:42: error: 'PHYS_OFFSET' undeclared (first use in this function)
 #define PHYS_PFN_OFFSET ((unsigned long)(PHYS_OFFSET >> PAGE_SHIFT))
arch/arm/include/asm/dma-mapping.h:104:9: note: in expansion of macro 'PHYS_PFN_OFFSET'
  return PHYS_PFN_OFFSET + dma_to_pfn(dev, *dev->dma_mask);

An easy workaround is for realview to define PHYS_OFFSET itself,
in the same way we define it for platforms that don't have a private
__virt_to_phys function.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Linus Walleij <linus.walleij@linaro.org>