rtc_hctosys expects RTCs in UTC (doc)

The RTC "hctosys" mechanism expects that RTC clock will use UTC, not local
time (e.g.  PST).  Say so in Kconfig and in the kernel message.

(Strictly speaking, the RTC clock should be tracking the POSIX epoch.  That's
not worth going into here.  Goofing timezones means clocks are wrong by many
hours; the POSIX-v-UTC differences just cost seconds.)

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>