mtd: nand: brcmnand: fix NAND timeout when accessing eMMC

When booting a board to NAND and accessing NAND while eMMC
transactions are occurring the NAND will sometimes timeout. This
is due to both NAND and eMMC controller sharing the same data bus
on BCMBCA chips. Fix is to extend NAND timeout to allow eMMC
transactions time to complete.

Signed-off-by: David Regan <dregan@broadcom.com>
Reviewed-by: William Zhang <william.zhang@broadcom.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>