ARM: OMAP2+: gpmc: fix: DT NAND child nodes not probed when MTD_NAND is built as module

Fixes: commit bc6b1e7b
       ARM: OMAP: gpmc: add DT bindings for GPMC timings and NAND

OMAP SoC(s) depend on GPMC controller driver to parse GPMC DT child nodes and
register them platform_device for NAND driver to probe later. However this does
not happen if generic MTD_NAND framework is built as module (CONFIG_MTD_NAND=m).

Therefore, when MTD/NAND and MTD/NAND/OMAP2 modules are loaded, they are unable
to find any matching platform_device and remain un-binded. This causes on board
NAND flash to remain un-detected.

This patch causes GPMC controller to parse DT nodes when
CONFIG_MTD_NAND=y || CONFIG_MTD_NAND=m

CC: <stable@vger.kernel.org> # 3.9.x+
Signed-off-by: Pekon Gupta <pekon@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>