mtd: gpmi: provide the option to use legacy bch geometry

Provide an option in DT to use legacy bch geometry, which compatible
with the 3.10 kernel bch setting. To enable the feature, adding
"fsl,legacy-bch-geometry" under gpmi-nand node.

NOTICE: The feature must be enabled/disabled in both u-boot and kernel.

Signed-off-by: Han Xu <han.xu@nxp.com>
Signed-off-by: Ye Li <ye.li@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>