mtd: nand: omap2: obtain memory from resource

gpmc initialization done by platform code now updates struct resource
with the address space alloted for nand. Use this interface to obtain
memory rather than relying on platform data field - phys_base.

Signed-off-by: Afzal Mohammed <afzal@ti.com>
Acked-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>