mtd: pxa3xx_nand: make scan procedure more clear

The previous probe function is some kind of big part.
This patch seperate the resource allocation to keep the probe process
more clear than before.

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>