rtc: mt6359: Add RTC hardware range and add support for start-year

Add the RTC hardware range parameters to enable the possibility
of using the `start-year` devicetree property which, if present,
will set the start_secs parameter by overriding the defaults
that this driver is setting;

To keep compatibility with (hence have the same date/time reading
as) the old behavior, set:
- range_min to 1900-01-01 00:00:00
- range_max to 2027-12-31 23:59:59 (HW year max range is 0-127)
- start_secs defaulting to 1968-01-02 00:00:00

Please note that the oddness of starting from January 2nd is not
a hardware quirk and it's done only to get the same date/time
reading as an RTC which time was set before this commit.

Also remove the RTC_MIN_YEAR_OFFSET addition and subtraction in
callbacks set_time() and read_time() respectively, as now this
is already done by the API.

Signed-off-by: AngeloGioacchino Del Regno <[e-mail]>
Reviewed-by: Macpaul Lin <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexandre Belloni <[e-mail]>

drivers/rtc/rtc-mt6397.c

···
 			goto exit;
 	} while (sec < tm->tm_sec);
 
-	/* HW register use 7 bits to store year data, minus
-	 * RTC_MIN_YEAR_OFFSET before write year data to register, and plus
-	 * RTC_MIN_YEAR_OFFSET back after read year from register
-	 */
-	tm->tm_year += RTC_MIN_YEAR_OFFSET;
-
 	/* HW register start mon from one, but tm_mon start from zero. */
 	tm->tm_mon--;
 	time = rtc_tm_to_time64(tm);
···
 	int ret;
 	u16 data[RTC_OFFSET_COUNT];
 
-	tm->tm_year -= RTC_MIN_YEAR_OFFSET;
 	tm->tm_mon++;
 
 	data[RTC_OFFSET_SEC] = tm->tm_sec;
···
 	tm->tm_mon = data[RTC_OFFSET_MTH] & RTC_AL_MTH_MASK;
 	tm->tm_year = data[RTC_OFFSET_YEAR] & RTC_AL_YEA_MASK;
 
-	tm->tm_year += RTC_MIN_YEAR_OFFSET;
 	tm->tm_mon--;
 
 	return 0;
···
 	int ret;
 	u16 data[RTC_OFFSET_COUNT];
 
-	tm->tm_year -= RTC_MIN_YEAR_OFFSET;
 	tm->tm_mon++;
 
 	mutex_lock(&rtc->lock);
···
 	device_init_wakeup(&pdev->dev, 1);
 
 	rtc->rtc_dev->ops = &mtk_rtc_ops;
+	rtc->rtc_dev->range_min = RTC_TIMESTAMP_BEGIN_1900;
+	rtc->rtc_dev->range_max = mktime64(2027, 12, 31, 23, 59, 59);
+	rtc->rtc_dev->start_secs = mktime64(1968, 1, 2, 0, 0, 0);
+	rtc->rtc_dev->set_start_time = true;
 
 	return devm_rtc_register_device(rtc->rtc_dev);
 }