mtd: pxa3xx-nand: handle PIO in threaded interrupt

Change the handling of the data stage in the driver : don't pump data in
the top-half interrupt, but rather schedule a thread for non dma cases.

This will enable latencies in the data pumping, especially if delays are
required. Moreover platform shall be more reactive as other interrupts
can be served while pumping data.

No throughput degradation was observed, at least on the zylonite
platform, while a slight degradation was being expected.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Tested-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>