drivers/rtc/Kconfig: place RTC_DRV_IMXDI and RTC_MXC under "on-CPU RTC drivers"

RTC_DRV_IMXDI and RTC_MXC are on-chip RTC modules, so move them under
"on-CPU RTC drivers" selection menu.

While at it change the dependency of RTC_DRV_IMXDI from ARCH_MX25 to
SOC_IMX25.

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