mx6sabresd: Convert to watchdog driver model

Commit 68dcbdd594 ("ARM: imx: Add weak default reset_cpu()") caused
the 'reset' command in U-Boot to not cause a board reset.

Fix it by switching to the watchdog driver model via sysreset, which
is the preferred method for implementing the watchdog reset.

Signed-off-by: Fabio Estevam <festevam@denx.de>

arch/arm/dts/imx6qdl-sabresd-u-boot.dtsi

@@ -9,6 +9,11 @@
 	aliases {
 		mmc1 = &usdhc3;
 	};
+	wdt-reboot {
+		compatible = "wdt-reboot";
+		wdt = <&wdog2>;
+		bootph-pre-ram;
+	};
 };
 
 &usdhc3 {
@@ -18,3 +23,7 @@
 &pinctrl_usdhc3 {
 	bootph-pre-ram;
 };
+
+&wdog2 {
+	bootph-pre-ram;
+};

configs/mx6sabresd_defconfig

@@ -104,6 +104,8 @@ CONFIG_DM_REGULATOR_FIXED=y
 CONFIG_DM_REGULATOR_GPIO=y
 CONFIG_DM_SERIAL=y
 CONFIG_MXC_UART=y
+CONFIG_SYSRESET=y
+CONFIG_SYSRESET_WATCHDOG=y
 CONFIG_SPI=y
 CONFIG_DM_SPI=y
 CONFIG_MXC_SPI=y
@@ -130,3 +132,4 @@ CONFIG_IMX_HDMI=y
 CONFIG_SPLASH_SCREEN=y
 CONFIG_SPLASH_SCREEN_ALIGN=y
 CONFIG_BMP_16BPP=y
+CONFIG_IMX_WATCHDOG=y