ARM: omap2+: gpmc-nand: Use dynamic platform_device_alloc()

GPMC controller supports up to 8 memory devices connected to it.
Since there is one statically allocated "struct platform_device
gpmc_nand_device" it is not possible to configure the system to
use more than one NAND device connected to the GPMC. This
modification makes it possible to use up to 8 NAND devices
connected to the GPMC controller.

Signed-off-by: Rostislav Lisovy <lisovy@merica.cz>
Signed-off-by: Roger Quadros <rogerq@ti.com>