rtc: fix coh901331 startup crash

The rtc_device_register() call has changed semantics so that it
will immediately call out to rtc_read_alarm() and since the
callbacks require the drvdata to be set, we need to set it before
the registration call to avoid NULL dereference.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>