dt-bindings: clk: qcom: Document MSM8976 Global Clock Controller

Document the required properties and firmware clocks for gcc-msm8976 to
operate nominally, and add header definitions for referencing the clocks
from firmware.

Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@somainline.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20211208091036.132334-2-marijn.suijten@somainline.org