mtd: nand: Making MTD_NAND_OMAP2 depend on ARCH_OMAP2PLUS

Making  MTD_NAND_OMAP2 depend on ARCH_OMAP2PLUS instead of
oring with ARCH2/3/4.

Reported-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>