rtc kconfig: point out need for static linkage

Various people have expressed surprise that their modular RTC drivers don't
seem to work for initializing the system time at boot.  To help avoid such
unpleasantness, make the Kconfig text point out that the driver probably
needs to be statically linked.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Acked-by: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>