mtd: nand: omap: read nand using register address

Now that gpmc-nand registers are available in driver, use it
to read nand data.

"65b97cf  mtd: nand: omap2: handle nand on gpmc" modified all
other instances. After initial versions of that patch, a new
change added reading nand data using gpmc exposed function.
In the final version this change was not taken care.

Signed-off-by: Afzal Mohammed <afzal@ti.com>