arm: dts: imx6qdl-u-boot: Enable spba-bus@2000000 simple bus

spba-bus has a few nodes under it including the UART1 and
some ESPI buses.  In order to use them in SPL, the
u-boot,dm-spl flag needs to be added to the spba-bus@2000000
container.

Signed-off-by: Adam Ford <aford173@gmail.com>
diff --git a/arch/arm/dts/imx6qdl-u-boot.dtsi b/arch/arm/dts/imx6qdl-u-boot.dtsi
index 45ae2fa..0aa29e3 100644
--- a/arch/arm/dts/imx6qdl-u-boot.dtsi
+++ b/arch/arm/dts/imx6qdl-u-boot.dtsi
@@ -9,6 +9,9 @@
 
 		aips-bus@2000000 {
 			u-boot,dm-spl;
+			spba-bus@2000000 {
+				u-boot,dm-spl;
+			};
 		};
 
 		aips-bus@2100000 {