imx/mx3: rename clock.c and iomux.c to {clock,iomux}-imx31.c

imx/mx3: rename clock.c and iomux.c to {clock,iomux}-imx31.c

These files are only used on imx31.  This makes the name consistent with
the imx35 files.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc: Sascha Hauer <kernel@pengutronix.de>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Daniel Mack <daniel@caiaq.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Valentin Longchamp <valentin.longchamp@epfl.ch>