spi: spi-zynqmp-gqspi: Update driver to use spi-mem framework

Updated Zynqmp qspi controller driver to use spi-mem framework.
Signed-off-by: NAmit Kumar Mahapatra <amit.kumar-mahapatra@xilinx.com>
Signed-off-by: NMichal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/490a7642a975f4d3dd9618304e9e45f7e2414661.1600931476.git.michal.simek@xilinx.comSigned-off-by: NMark Brown <broonie@kernel.org>