dt-bindings: clock: thead: Add TH1520 VO clock controller

Add device tree bindings for the TH1520 Video Output (VO) subsystem
clock controller. The VO sub-system manages clock gates for multimedia
components including HDMI, MIPI, and GPU.

Document the VIDEO_PLL requirements for the VO clock controller, which
receives its input from the AP clock controller. The VIDEO_PLL is a
Silicon Creations Sigma-Delta (integer) PLL typically running at 792 MHz
with maximum FOUTVCO of 2376 MHz.

This binding complements the existing AP sub-system clock controller
which manages CPU, DPU, GMAC and TEE PLLs.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Michal Wilczynski <m.wilczynski@samsung.com>
Reviewed-by: Drew Fustini <drew@pdp7.com>
Signed-off-by: Drew Fustini <drew@pdp7.com>