rtc: clarify the RTC offset correction

The RTC offset correction documentation is not very clear about the
exact relationship between "offset" and the effect it has on the RTC.
Supplement the documentation with an equation giving the relationship.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>