dt: bindings: brcmnand: Add support for flash-edu

Adding support for EBI DMA unit (EDU).

Signed-off-by: Kamal Dasu <kdasu.kdev@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200122213313.35820-2-kdasu.kdev@gmail.com