spi: spi-ep93xx: convert to the queued driver infrastructure

The SPI core provides infrastructure for standard message queueing. Use
that instead of handling it in the driver.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Acked-by: Mika Westerberg <mika.westerberg@iki.fi>
Signed-off-by: Mark Brown <broonie@linaro.org>