spi: imx: Implement set_speed

The set_speed() callback should configure the bus speed, make it so.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

drivers/spi/mxc_spi.c

@@ -662,7 +662,10 @@ static int mxc_spi_release_bus(struct udevice *dev)
 
 static int mxc_spi_set_speed(struct udevice *bus, uint speed)
 {
-	/* Nothing to do */
+	struct mxc_spi_slave *mxcs = dev_get_plat(bus);
+
+	mxcs->max_hz = speed;
+
 	return 0;
 }