[MTD] NAND: Add suspend/resume functionality

The changes introduced allow to suspend/resume NAND flash.
A new state (FL_PM_SUSPENDED) is introduced, as well as
routines for mtd->suspend and mtd->resume to put the flash in
suspended state from software pov.

Signed-off-by: Vitaly Wool <vwool@ru.mvista.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>