feat: Add shared layout .dtsi files to promote reuse.

* Add Corne and Ferris shared layouts, and update the respective
  shields to use them.
* Add a sample Hummingbird physical layout for testing posix xiao.
* Add Sofle physical layout as an additional reference.

app/boards/arm/ferris/ferris_rev02.dts

@@ -10,6 +10,12 @@
 
 #include <dt-bindings/zmk/matrix_transform.h>
 
+#include <layouts/cuddlykeyboards/ferris.dtsi>
+
+&cuddlykeyboards_ferris_layout {
+    transform = <&transform>;
+};
+
 / {
     model = "Ferris rev0.2";
     compatible = "ferris,rev02", "st,stm32f072";
@@ -18,7 +24,6 @@
         zephyr,sram = &sram0;
         zephyr,flash = &flash0;
         zmk,kscan = &kscan;
-        zmk,matrix-transform = &transform;
         /* TODO: Enable once we support the IC for underglow
         zmk,underglow = &led_strip;
          */
@@ -114,6 +119,9 @@ zephyr_udc0: &usb {
 
     pinctrl-0 = <&usb_dm_pa11 &usb_dp_pa12>;
     pinctrl-names = "default";
+
+    // Overridden to lower RAM usage.
+    num-bidir-endpoints = <4>;
 };
 
 &clk_hsi {