spi: cadence_qspi: Move to spi-mem framework

Current Cadence QSPI driver has few limitations. It assumes all read
operations to be in Quad mode and thus does not support SFDP parsing.
Also, adding support for new mode such as Octal mode would not be
possible with current configuration. Therefore move the driver over to spi-mem
framework. This has added advantage that driver can be used to support
SPI NAND memories too.
Hence, move driver over to new spi-mem APIs.

Please note that this gets rid of mode bit setting done when
CONFIG_SPL_SPI_XIP is defined as there does not seem to be any user to
that config option.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Tested-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Acked-by: Jagan Teki <jagan@amarulasolutions.com>