rtc: Add RTC range

Add a way for drivers to inform the core of the supported date/time range.
The core can then check whether the date/time or alarm is in the range
before calling ->set_time, ->set_mmss or ->set_alarm. It returns -ERANGE
when the time is out of range.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

rtc: nvmem: allow registering the nvmem device before the rtc

Switch the parent of the nvmem device to the parent of the rtc device so it
can be registered before the RTC.

This is a small change in the ABI as the nvmem moves out of the
/sys/class/rtc/rtcX folder to be under the parent device folder (that is
where the previous nvram files where registered).

However, it is still available under its correct location,
/sys/bus/nvmem/devices which is the one that should be used by userspace
applications.

The other benefit is that the nvmem device can stay registered even if the
rtc registration fails. Or it is possible to not register the rtc if the
nvmem registration failed.

Finally, it makes a lot of sense for devices that actually have different
i2c or spi addresses for the RTC and the EEPROM. That is basically how it
would end up when using MFD or even completely separate devices.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

rtc: documentation: correct nvmem date and version

The nvmem interface appaeared in linux 4.13

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

Documentation: rtc: add sysfs file permissions

Annotate the sysfs interface of rtc with file specific permissions
(RO/RW).

Signed-off-by: Aishwarya Pant <aishpant@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

Documentation: rtc: move sysfs documentation to ABI

The sysfs documentation should be located under Documentation/ABI.

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