mtd: nand: mxs_nand: use structure for BCH geometry

mtd: nand: mxs_nand: use structure for BCH geometry

Calculate BCH geometry at start and store the information in
a structure. This avoids recalculation on every page access
and allows to calculate ECC relevant information in one place.
This patch does not change ECC layout or driver behavior in
any way.

The patch aligns the driver somewhat with the Linux GPMI NAND
driver which drives the same IP.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>