mxc_nand: add V1_V2 namespace to registers

This prepares the driver for v3 support. The v3 controller
has a completely different register layout, so add a V1_V2_
namespace to the register defines to avoid confusion with
the v3 regs.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>