mtd: nand: added nand_shutdown

Add nand_shutdown to wait for current nand operations to finish and prevent
further operations by changing the nand flash state to FL_SHUTDOWN.

This is addressing a problem observed during reboot tests using UBIFS
root file system: NAND erase operations that are in progress during
system reboot/shutdown are causing partial erased blocks. Although UBI should
be able to detect and recover from this error, this change will avoid
the creation of partial erased blocks on reboot in the middle of a NAND erase
operation.

Signed-off-by: Scott Branden <sbranden@broadcom.com>
Tested-by: Scott Branden <sbranden@broadcom.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>