arm/imx/iomux-mx25: unify style and comment cleanup

- use __MACH_IOMUX_MX25_H__ as header protector analogous to
  <mach/mx...h>
- remove doxygen comments
- remove #error about mach/iomux.h which is unused on mx25
- remove #ifndef __ASSEMBLY__ which is unneeded here

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