ARM split - detect USB to select master/slave (#6424)

* Initial split refactor to allow usb master detection

* Add split USB detect docs

* Add SPLIT_USB_DETECT demo mode limitation

* fix rebase issues

* clang-format

quantum/split_common/split_util.h

@@ -8,3 +8,4 @@
 extern volatile bool isLeftHand;
 
 void matrix_master_OLED_init(void);
+void keyboard_split_setup(void);