krane: bind the mtu3 SSUSB container so its xhci child is scanned

DM scans only the immediate subnodes of a bound node; recursion into
children happens when a bound bus driver runs dm_scan_fdt_dev. Nothing
in U-Boot binds the "mediatek,mtu3" ssusb@11201000 node, so its xhci
child (usb@11200000) was never bound, no UCLASS_USB device existed and
usb start() reported "No USB controllers found".

Add "simple-mfd" to the ssusb compatible in the -u-boot.dtsi overlay
so the generic simple-bus driver binds the node and scans its children.

arch/arm/dts/mt8183-kukui-krane-sku176-u-boot.dtsi

@@ -12,8 +12,18 @@
  * and "ippc" register names and a phys list on the xHCI node itself.
  * Add the missing ippc address (taken from the ssusb node's "ippc"
  * reg entry) and the port phys there.
+ *
+ * DM only scans the subnodes of nodes that a driver binds (bus drivers
+ * recurse via dm_scan_fdt_dev). Nothing binds "mediatek,mtu3", so the
+ * xHCI child would never be bound and usb_start() would report "No USB
+ * controllers found". Adding "simple-mfd" makes the generic simple-bus
+ * driver bind the ssusb node and scan its children.
  */
 
+&ssusb {
+	compatible = "mediatek,mt8183-mtu3", "mediatek,mtu3", "simple-mfd";
+};
+
 &usb_host {
 	reg = <0x0 0x11200000 0x0 0x1000>,
 	      <0x0 0x11203e00 0x0 0x0100>;