ARM i.MX50: Rename devices-mx50.h

There are devices-imx51.h and devices-imx53.h under
arch/arm/mach-mx5 directory. So, had better rename
devices-mx50.h to devices-imx50.h to follow the same
naming convention with imx51 and imx53 part.

Signed-off-by: Jason Liu <r64343@freescale.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>