rtc: explicitly set tm_sec = 0 for drivers with minute accurancy

Since all time members of the alarm data is initialized to -1 the drivers
are responsible to set the tm_sec member to 0.

Fixes: d68778b8 ("rtc: initialize output parameter for read alarm to "uninitialized"")
Signed-off-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>