rtc: sunxi: Depend on platforms sun4i/sun7i that actually have the rtc

Now that we have Kconfig options for individual sunxi platforms, let
the rtc-sunxi driver depend on the platforms that actually have this
hardware, sun4i and sun7i.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>