clk: sunxi: Add a dummy clock driver for the RTC

The 32kHz clock ("LOSC") on sunxi SoCs is provided by the RTC. It is
used, among other things, by the XHCI controller in the H6. To be able
to call clk_get_bulk() on the XHCI controller, some device needs to
provide all referenced clocks.

Since LOSC is a fixed-rate always-on clock, implementation is trivial.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>