ext: mcux: add MCR20Overwrites.h

Add overwrites header file for MCR20 Register values.

Overwrite values are non-default register values that
configure the transceiver device to a more optimally
performing posture.
(Excerpt from the documentation part of MCR20Overwrites.h)

Origin: Kinetis MKW2xD and MCR20A Connectivity Software
URL: https://www.nxp.com/


(The complete URL can be found in the ext/hal/nxp/mcux/README)
Revision: (REV 1.0.0)
Maintained-by: External

Change-Id: Id96db2ad70279c0b11d749d65e9148244d8fdc2f
Signed-off-by: Johann Fischer <j.fischer@phytec.de>