arch: mips: brcm: Add 7425 flash-edu support

Nand controller v5.0 and v6.0 have nand edu blocks that enable
dma nand flash transfers. This allows for faster read and write
access.

Signed-off-by: Kamal Dasu <kdasu.kdev@gmail.com>
Acked-by: Paul Burton <paulburton@kernel.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200122213313.35820-3-kdasu.kdev@gmail.com