clk: meson: axg: add Video Clocks

Add the clocks entries used in the video clock path, the clock path is
doubled to permit having different synchronized clocks for different parts
of the video pipeline.

The AXG only has a single ENCL CTS clock and even if VCLK exist along
VCLK2, only VCLK2 is used since it clocks the MIPI DSI IP directly.

All dividers are flagged with CLK_GET_RATE_NOCACHE, and all gates are
flagged with CLK_IGNORE_UNUSED since they are currently directly handled by
the Meson DRM Driver.  Once the DRM Driver is fully migrated to using the
Common Clock Framework to handle the video clock tree, the
CLK_GET_RATE_NOCACHE and CLK_IGNORE_UNUSED will be dropped.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://lore.kernel.org/r/20200915124553.8056-4-narmstrong@baylibre.com