board: sl28: fix USB0

Since commit 61ff1328 ("board: sl28: move to OF_UPSTREAM") USB0 is
broken because the former u-boot soc dtsi was setting dr_mode to "host"
but the linux device tree isn't. That is because linux fully supports
OTG but u-boot doesn't. Therefore, u-boot only ever enabled host mode
and never OTG mode. Add it to our board "-u-boot.dtsi" to fix it.

Fixes: 61ff1328 ("board: sl28: move to OF_UPSTREAM")
Reported-by: Heiko Thiery <heiko.thiery@gmail.com>
Signed-off-by: Michael Walle <mwalle@kernel.org>
Tested-by: Heiko Thiery <heiko.thiery@gmail.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>

arch/arm/dts/fsl-ls1028a-kontron-sl28-u-boot.dtsi

@@ -203,3 +203,7 @@
 &sysclk {
 	bootph-all;
 };
+
+&usb0 {
+	dr_mode = "host";
+};