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 <chunyan.zhang@unisoc.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lkml.kernel.org/r/20200304072730.9193-3-zhang.lyra@gmail.comSigned-off-by: Stephen Boyd <sboyd@kernel.org>

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

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