doc: dt: mtd: brcmnand: Add brcm,bcm6368-nand device tree binding

Add device tree binding for NAND on the BCM6368.

The BCM6368 has a NAND interrupt register with combined status and enable
registers. It also requires a clock, so add an optional clock to the
common brcmnand binding.

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