mtd: spi-nor: fsl-quadspi: add imx7d support

Support i.mx7d.
quadspi in i.mx7d increase rxfifo.
require fill at least 16byte to trigger data transfer.

Signed-off-by: Frank Li <Frank.Li@freescale.com>
Signed-off-by: Han Xu <han.xu@freescale.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>