imx28: avoid num_cs and spi_max_frequency build errors

Avoid the following build errors after the device tree sync:

drivers/spi/mxs_spi.c: In function ‘mxs_spi_probe’:
drivers/spi/mxs_spi.c:327:25: error: ‘struct dtd_fsl_imx23_spi’ has no
 member named ‘spi_max_frequency’
  327 |  priv->max_freq = dtplat->spi_max_frequency;
      |                         ^~
drivers/spi/mxs_spi.c:328:23: error: ‘struct dtd_fsl_imx23_spi’ has no
 member named ‘num_cs’
  328 |  plat->num_cs = dtplat->num_cs;
      |                       ^~

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>

arch/arm/dts/imx28-xea-u-boot.dtsi

@@ -42,5 +42,7 @@
 };
 
 &ssp3 {
+	num-cs = <2>;
+	spi-max-frequency = <40000000>;
 	u-boot,dm-spl;
 };