Remove unreferenced global function imx_gpt_get_timerfreq() and do some
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.