dt-bindings: clock: Introduce QCOM Graphics clock bindings

Add device tree bindings for graphics clock controller for
Qualcomm Technology Inc's SDM845 SoCs.

Signed-off-by: Amit Nischal <anischal@codeaurora.org>
Reviewed-by: Rob Herring <robh@kernel.org>
[sboyd@kernel.org: Add input clocks property]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>