* Previous version of multiple overrides of the default value of ZMK_USB
were problematic. Move to using board _defconfig files for the
defaults for those, along with proper `depends on` for ZMK_USB that
accounts for split and split roles.
- Add ZMK_KSCAN_MATRIX_POLLING config flag to Kconfig
- Update matrix driver code to use the above flag to conditionally add the handling code for polling operations.
