cleanups enabled by that:
- The only thing left in imx_gptvar.h was the softc, which IMO never
should have been in there at all. Move it into the driver, and
delete the header file.
- Remove several unneeded #includes from the driver.
- Change imx_gpt_softc from global to static (it's used by DELAY()), and
don't redundantly static-initialize it to NULL.