dt-bindings: clk: g12a-clkc: add VDEC clock IDs

Expose the three clocks related to the video decoder.

Signed-off-by: Maxime Jourdan <mjourdan@baylibre.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://lkml.kernel.org/r/20190319101138.27520-2-mjourdan@baylibre.com