spi: spi-mem: Add support for Zynq QSPI controller

Add support for QSPI controller driver used by Xilinx Zynq SOC.

Signed-off-by: Naga Sureshkumar Relli <naga.sureshkumar.relli@xilinx.com>
Signed-off-by: Mark Brown <broonie@kernel.org>