mtd: nand: omap: Synchronize the access to the ECC engine

The AM335x Technical Reference Manual (spruh73j.pdf) says
"Because the ECC engine includes only one accumulation context,
it can be allocated to only one chip-select at a time ... "
(7.1.3.3.12.3). Since the commit 97a288ba ("ARM: omap2+:
gpmc-nand: Use dynamic platform_device_alloc()") gpmc-nand
driver supports multiple NAND flash devices connected to
the single controller.
Use global 'struct nand_hw_control' among multiple NAND
instances to synchronize the access to the single ECC Engine.

Tested with custom AM335x board using 2x NAND flash chips.

Signed-off-by: Rostislav Lisovy <lisovy@merica.cz>
Acked-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>