mtd: spi-nor: fsl-quadspi: add big-endian support

Add R/W functions for big- or little-endian registers:
The qSPI controller's endian is independent of the CPU core's endian.
So far, the qSPI have two versions for big-endian and little-endian.
Signed-off-by: Yuan Yao <yao.yuan@nxp.com>
Acked-by: Han xu <han.xu@freescale.com>
Acked-by: Han xu <han.xu@nxp.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>