Cosmetic fixes to RTC subsystem's Kconfig

Add adds a warning if a potentially conflicting RTC option has been
selected and makes some other cosmetic fixes to the Kconfig.

Signed-off-by: Alessandro Zummo <a.zummo@towertech.it>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>