spi: zynqmp_gqspi: Fix dma alignment issue

DMA is aligned to ARCH_DMA_MINALIGN(64 bytes), but as per spec, alignment
required is 4bytes only. Change DMA alignment from ARCH_DMA_MINALIGN to
GQSPI_DMA_ALIGN. Remove alignment of data length in non-exponential case.

Some minor improvements in the initialization to initialize gen_fifo
threshold and disable qspi controller while setting config register.

Signed-off-by: Ashok Reddy Soma <ashok.reddy.soma@xilinx.com>
Tested-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>