spi: ti_qspi: Convert to spi-mem ops

Convert driver to use  spi-mem ops in order to support accelerated MMIO
flash interface in generic way and for better performance.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Jagan Teki <jagan@amarulasolutions.com>