mtd: pxa3xx_nand: condense the flash definition

Adding a new flash definition would need less code.
Keep the platform passing flash definition method.
If one flash is both defined in platform data and builtin table,
driver would select the one from platform data first.

By this way, platform could select the timing most suit for itself,
not need to follow the common settings.

Signed-off-by: Lei Wen <leiwen@marvell.com>
Signed-off-by: Haojian Zhuang <haojian.zhuang@marvell.com>
Acked-by: Eric Miao <eric.y.miao@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>