mtd: brcmnand: Add support for the BCM6368

The BCM6368 has a NAND interrupt register with combined status and enable
registers.

As the BCM6328, BCM6362 and BCM6368 all use v2.1 controllers, the first
variant that will work with this driver is the BCM63268 using a v4.0
controller.

Set up the device by disabling and acking all interrupts, then handle
the CTRL_READY interrupt.

Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>