mtd: pxa3xx_nand: rework irq logic

Enable all irq when we start the nand controller, and
put all the transaction logic in the pxa3xx_nand_irq.

By doing this way, we could dramatically increase the
performance by avoid unnecessary delay.

Signed-off-by: Lei Wen <leiwen@marvell.com>
Signed-off-by: Haojian Zhuang <haojian.zhuang@marvell.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>