rtc: sun6i: Allow using as wakeup source from suspend

rtc: sun6i: Allow using as wakeup source from suspend

This patch allows userspace to set up wakeup alarms on any RTC handled by the
sun6i driver, and adds the necessary PM operations to allow resuming from
suspend when the configured wakeup alarm fires a IRQ. Of course, that the
device actually resumes depends on the suspend state and how a particular
hardware reacts to it, but that is out of scope for this patch.

I've tested these changes on a Pine H64 model B, which contains a
Allwinner H6 SoC, with the help of CONFIG_PM_TEST_SUSPEND kernel option.
These are the interesting outputs from the kernel and commands which
show that it works. As every RTC handled by this driver is largely the
same, I think that it shouldn't introduce any regression on other SoCs,
but I may be wrong.

[    1.092705] PM: test RTC wakeup from 'freeze' suspend
[    1.098230] PM: suspend entry (s2idle)
[    1.212907] PM: suspend devices took 0.080 seconds
(The SoC freezes for some seconds)
[    3.197604] PM: resume devices took 0.104 seconds
[    3.215937] PM: suspend exit

[    1.092812] PM: test RTC wakeup from 'mem' suspend
[    1.098089] PM: suspend entry (deep)
[    1.102033] PM: suspend exit
[    1.105205] PM: suspend test failed, error -22

In any case, the RTC alarm interrupt gets fired as exptected:

$ echo +5 > /sys/class/rtc/rtc0/wakealarm && sleep 5 && grep rtc /proc/interrupts
 29:          1          0          0          0     GICv2 133 Level     7000000.rtc

Signed-off-by: Alejandro González <alejandro.gonzalez.correo@gmail.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Link: https://lore.kernel.org/r/20190821210056.11995-1-alejandro.gonzalez.correo@gmail.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>