clk: sunxi-ng: h6-r: Add RTC gate clock

The H6 and H616 feature an (undocumented) bus clock gate for accessing
the RTC registers. This seems to be enabled at reset (or by the BootROM),
so we got away without it so far, but exists regardless.
Since the new RTC clock binding for the H616 requires this "bus" clock
to be specified in the DT, add this to R_CCU clock driver and expose it
on the DT side with a new number.
We do this for both the H6 and H616, but mark it as IGNORE_UNUSED, as we
cannot reference it in any H6 DTs.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Link: https://lore.kernel.org/r/20220428230933.15262-2-andre.przywara@arm.com