ARM: S5PV210: Fix on MAX_PHYSMEM_BITS on S5PV210/S5PC110.

This patch fixes on MAX_PHYSMEM_BITS for Sparsemem on S5PV210/S5PC110.
Because physical memory can be located from 0x20000000 to 0x7fffffff,
so MAX_PHYSMEM_BITS is 31 and not 32.

Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>