mtd: mxc_nand: Select the driver via ARCH_MXC

With device tree support in place, we should not use IMX_HAVE_PLATFORM_MXC_NAND
as a dependency for selecting the mxc_nand driver.

Use ARCH_MXC symbol instead, so that the driver can be even selected when a single device-tree
machine is selected.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>