spi: spi-uclass: Read chipselect and restrict capabilities

Read chipselect properties from DT which are populated using 'reg'
property and save it in plat->cs[] array for later use.

Also read multi chipselect capability which is used for
parallel-memories and return errors if they are passed on using DT but
driver is not capable of handling it.

Signed-off-by: Ashok Reddy Soma <ashok.reddy.soma@amd.com>
Signed-off-by: Venkatesh Yadav Abbarapu <venkatesh.abbarapu@amd.com>
diff --git a/drivers/spi/gxp_spi.c b/drivers/spi/gxp_spi.c
index 70d76ac..3ee369c 100644
--- a/drivers/spi/gxp_spi.c
+++ b/drivers/spi/gxp_spi.c
@@ -87,7 +87,7 @@
 		value = readl(priv->base + OFFSET_SPIMCFG);
 		value &= ~(1 << 24);
 		/* set chipselect */
-		value |= (slave_plat->cs << 24);
+		value |= (slave_plat->cs[0] << 24);
 
 		/* addr reg and addr size */
 		if (len >= 4) {