imx: imx6sx-sabreauto: convert to use DM QSPI driver

To support DM QSPI driver:
 - Add -u-boot.dtsi to modify n25q256a@0 and n25q256a@1 compatible string
   to "spi-flash" and add "num-cs" property.
 - Enable DM SPI and DM SPI FLASH configurations
 - Remove iomux settings of qspi1 in board codes which is not needed
   for DM driver.

Signed-off-by: Ye Li <ye.li@nxp.com>
diff --git a/arch/arm/dts/imx6sx-sabreauto-u-boot.dtsi b/arch/arm/dts/imx6sx-sabreauto-u-boot.dtsi
new file mode 100644
index 0000000..f5c68d7
--- /dev/null
+++ b/arch/arm/dts/imx6sx-sabreauto-u-boot.dtsi
@@ -0,0 +1,16 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Copyright 2018 NXP
+ */
+
+&qspi1 {
+	num-cs = <2>;
+
+	flash0: n25q256a@0 {
+		compatible = "spi-flash";
+	};
+
+	flash1: n25q256a@1 {
+		compatible = "spi-flash";
+	};
+};