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/mtd/spi/sandbox.c b/drivers/mtd/spi/sandbox.c
index 2d5a16b..e5ebc34 100644
--- a/drivers/mtd/spi/sandbox.c
+++ b/drivers/mtd/spi/sandbox.c
@@ -138,7 +138,7 @@
 		return ret;
 	}
 	slave_plat = dev_get_parent_plat(dev);
-	cs = slave_plat->cs;
+	cs = slave_plat->cs[0];
 	debug("found at cs %d\n", cs);
 
 	if (!pdata->filename) {