rtc: rename CONFIG_RTC_MXC to CONFIG_RTC_DRV_MXC

In order to keep consistency with other rtc drivers,rename CONFIG_RTC_MXC
to CONFIG_RTC_DRV_MXC.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Wolfram Sang <w.sang@pengutronix.de>
Cc: Alessandro Zummo <a.zummo@towertech.it>
[akpm@linux-foundation.org: fix missed arch/arm/configs/imx_v6_v7_defconfig]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>