spi: cadence_quadspi: Enable QUAD mode based on DT data

Instead of relying on CONFIG_SPI_FLASH_QUAD to be defined to enable QUAD
mode, make use of mode_rx field of dm_spi_slave_platdata to determine
whether to enable or disable QUAD mode. This is necessary to support
muliple SPI controllers where one of them may not support QUAD mode.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Tested-by: Marek Vasut <marex@denx.de>
Acked-by: Marek Vasut <marex@denx.de>
Reviewed-by: Jagan Teki <jteki@openedev.com>