rtc: sun6i: Add support for all known pre-H6 variants

There are different variants to the RTC hardware first seen on sun6i
(A31). The differences we care about in this driver are the clock rate
for the internal oscillator, prescalers, and the presence of an external
clock output.

This patch adds support for all the known pre-H6 base compatibles using
the variants data structure previously introduced.

Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Tested-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>