rtc: mcp795: add alarm support.

This patch adds alarm support. This allows to configure the chip
to generate an interrupt when the alarm matches current time value.
Alarm can be programmed up to one year in the future
and is accurate to the second.

Signed-off-by: Emil Bartczak <emilbart@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>