spi: ti-qspi: Implement the spi_mem interface

The spi_mem interface is meant to replace the spi_flash_read() one.
Implement the ->exec_op() method so that we can smoothly get rid of the
old interface.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Vignesh R <vigneshr@ti.com>
Tested-by: Vignesh R <vigneshr@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>