Documentation: fsl-quadspi: Add optional properties

Add optional properties for QSPI:
big-endian
if the register is big endian on this platform.

Signed-off-by: Yuan Yao <yao.yuan@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Han xu <han.xu@nxp.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>