[Core] Split support for pointing devices. (#15304)

* Draft implementation

* formatting

* fix combined buttons

* remove pimoroni throttle

* sync pointing on a throttle loop with checksum

* no longer used

* doh

Co-authored-by: Drashna Jaelre <[email protected]>

* switch pimoroni to a cpi equivalent

* add cpi support

* allow user modification of seperate mouse reports

* a little tidy up

* add *_RIGHT defines.

* docs

* doxygen comments

* basic changelog

* clean up pimoroni

* small doc fixes

* Update docs/feature_pointing_device.md

Co-authored-by: Drashna Jaelre <[email protected]>

* performance tweak if side has usb

* Don't run init funtions on wrong side

* renamed some variables for consistency

* fix pimoroni typos

* Clamp instead of OR

* Promote combined values to uint16_t

* Update pointing_device.c

Co-authored-by: Drashna Jaelre <[email protected]>
Co-authored-by: Nick Brassel <[email protected]>

docs/ChangeLog/20220226/PR15304.md

@@ -0,0 +1,13 @@
+### Split Common core now supports Pointing Devices ([#15304](https://github.com/qmk/qmk_firmware/pull/15304))
+
+Pointing devices can now be shared across a split keyboard with support for a single pointing device or a pointing device on each side.
+
+This feature can be enabled with `#define SPLIT_POINTING_ENABLE` and one of the following options:
+
+| Setting                   | Description                        |
+|---------------------------|------------------------------------|
+|`POINTING_DEVICE_LEFT`     | Pointing device on the left side   |
+|`POINTING_DEVICE_RIGHT`    | Pointing device on the right side  |
+|`POINTING_DEVICE_COMBINED` | Pointing device on both sides      |
+
+See the [Pointing Device](../feature_pointing_device.md) documentation for further configuration options.