Add information about the new i.MXRT1166 BSP. Rework some parts that
have been changed during or as preparation for that variant:
* The BSP now adapts to the Chip variant. It's no longer necessary to
overwrite the PLL settings in an application.
* Improve documentation on how to adapt to different boards.
* Add Update the i.MXRT chapter so that it represents the new i.MXRT1166
BSP.
* Add information about mcux-sdk and how to handle it.