arm64: dts: sc7180: Add the sleep_clk to gcc-sc7180 node

The bindings say that we're supposed to have this in the node.  Who am
I to argue with the bindings?

Fixes: 90db71e4 ("arm64: dts: sc7180: Add minimal dts/dtsi files for SC7180 soc")
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20200129144432.1.Ie36f0532f67b0221c1e48e7cf6863a2738716a54@changeid


Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>