general: icm4268x: Refactor ICM42688 driver to ICM4268X

As a first step to enable the similar variants (e.g: ICM42686),
refactor common functionality into icm4268x files. As a result,
applications using the icm42688 will need to have both compatible
properties: "invensense,icm42688" and "invensense,icm4268x" defined.
In-tree boards have been modified to comply with this pattern.

This patch does not contain functional changes. The driver should
work the same as before.

Signed-off-by: Luis Ubieda <luisf@croxel.com>