spi-imx: Implements handling of the SPI_READY mode flag.

This patch implements consideration of the SPI_READY mode flag as
defined in spi.h. It extends the device tree bindings to support
the values defined by the reference manual for the DRCTL field.

Thus supporting edge-triggered and level-triggered bursts.

Signed-off-by: Leif Middelschulte <Leif.Middelschulte@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>