spi: spi-zynqmp-gqspi: add mutex locking for exec_op

The spi-mem framework has no locking to prevent ctlr->mem_ops->exec_op
from concurrency. So add the locking to zynqmp_qspi_exec_op.

Fixes: 1c26372e ("spi: spi-zynqmp-gqspi: Update driver to use spi-mem framework")
Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>
Reviewed-by: Amit Kumar Mahapatra <amit.kumar-mahapatra@xilinx.com>
Link: https://lore.kernel.org/r/20210408040223.23134-3-quanyang.wang@windriver.com


Signed-off-by: Mark Brown <broonie@kernel.org>