imx: imx6sx-sdb: Enable DM QSPI driver

To support DM QSPI driver
 - Add spi0 and spi1 alias for qspi1 and qspi2.
 - Add -u-boot.dtsi to modify n25q256a@0 and n25q256a@1 compatible string
   to "spi-flash" and add "num-cs" property.
 - Enable DM SPI/QSPI relavent configurations
 - Remove iomux settings of qspi2 in board codes which is not needed
   for DM driver.
 - Add sf default settings. So running "sf probe" can detect the flash

Signed-off-by: Ye Li <ye.li@nxp.com>

arch/arm/dts/imx6sx-sdb-u-boot.dtsi

@@ -0,0 +1,16 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Copyright 2018 NXP
+ */
+
+&qspi2 {
+	num-cs = <2>;
+
+	flash0: n25q256a@0 {
+		compatible = "spi-flash";
+	};
+
+	flash1: n25q256a@1 {
+		compatible = "spi-flash";
+	};
+};

arch/arm/dts/imx6sx.dtsi

@@ -40,11 +40,13 @@
 		serial3 = &uart4;
 		serial4 = &uart5;
 		serial5 = &uart6;
-		spi0 = &ecspi1;
-		spi1 = &ecspi2;
-		spi2 = &ecspi3;
-		spi3 = &ecspi4;
-		spi4 = &ecspi5;
+		spi0 = &qspi1;
+		spi1 = &qspi2;
+		spi2 = &ecspi1;
+		spi3 = &ecspi2;
+		spi4 = &ecspi3;
+		spi5 = &ecspi4;
+		spi6 = &ecspi5;
 		usbphy0 = &usbphy1;
 		usbphy1 = &usbphy2;
 	};