dt-bindings: clock: meson8b: add the clock inputs

The clock controller on Meson8/Meson8b/Meson8m2 has three (known)
inputs:
- "xtal": the main 24MHz crystal
- "ddr_pll": some of the audio clocks use the output of the DDR PLL as
  input
- "clk_32k": an optional clock signal which can be connected to GPIOAO_6
  (which then has to be switched to the CLK_32K_IN function)

Add the inputs to the documentation so we can wire up these inputs in a
follow-up patch.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>