rtc: allow compilation of sun6i RTC for all sunxi SoCs

At the moment the "sun6i" RTC drivers depends on having two specific
SoC families selected.
The Allwinner A64 SoC has the same RTC, so extend the Kconfig option
to allow inclusion of the driver for all Allwinner SoCs.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>