rtc: armada38x: add support for trimming the RTC

Add support for trimming the RTC using the offset mechanism.  This RTC
supports two modes: low update mode and high update mode.  Low update
mode has finer precision than high update mode, so we use the low mode
where possible.

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