arm/imx/iomux-v1: rename header file

Addionally make iomux-mx*.h headers stand-alone and similar to iomux-v3
platform files should include their platform iomux header from now on.
For now iomux.h simply includes all iomux-v1 platform headers and so
provides compatibility until all files are converted.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>