dt-bindings: clk: sprd: rename the common file name sprd.txt to SoC specific

Only SC9860 clocks were described in sprd.txt, rename it with a SoC
specific name, so that we can add more SoC support.

Signed-off-by: Chunyan Zhang <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Stephen Boyd <[e-mail]>

Documentation/devicetree/bindings/clock/sprd.txt Documentation/devicetree/bindings/clock/sprd,sc9860-clk.txt

···
-Spreadtrum Clock Binding
+Spreadtrum SC9860 Clock Binding
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 Required properties: