One of the uses of the external clock outputs is to provide a stable
32768 Hz clock signal to WiFi and Bluetooth chips. On the R40, the RTC
has an internal RC oscillator that is muxed with the external crystal.

Allow setting the parent rate for the external clock outputs so that
requests for 32768 Hz get passed to the RTC's clock driver to mux in
the external crystal if it isn't already muxed correctly.

Fixes: cd030a78 ("clk: sunxi-ng: support R40 SoC")
Fixes: 01a7ea76 ("clk: sunxi-ng: r40: Force LOSC parent to RTC LOSC output")
Cc: <stable@kernel.org>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>