dt-bindings: clock: Document gcc bindings for SM8150

Document the global clock controller found on SM8150.

Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>
Signed-off-by: Taniya Das <tdas@codeaurora.org>
[vkoul: port to upstream and add external clocks
	split binding to this patch]]
Signed-off-by: Vinod Koul <vkoul@kernel.org>
Link: https://lkml.kernel.org/r/20190722074348.29582-5-vkoul@kernel.org


Signed-off-by: Stephen Boyd <sboyd@kernel.org>