mtd: rawnand: brcmnand: Set appropriate DMA mask

NAND controllers >= 7.0 with FLASH_DMA support physical addresses up to
40-bit, set an appropriate DMA mask for that purpose.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>