rtc: s35390a: implement reset routine as suggested by the reference

commit 8e6583f1b5d1f5f129b873f1428b7e414263d847 upstream.

There were two deviations from the reference manual: you have to wait
half a second when POC is active and you might have to repeat
initialization when POC or BLD are still set after the sequence.

Note however that as POC and BLD are cleared by read the driver might
not be able to detect that a reset is necessary. I don't have a good
idea how to fix this.

Additionally report the value read from STATUS1 to the caller. This
prepares the next patch.

Signed-off-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>