csky: remove deprecated arch/csky/boot/dts/include/dt-bindings

Having a symbolic link arch/*/boot/dts/include/dt-bindings was
deprecated by commit d5d332d3 ("devicetree: Move include
prefixes from arch to separate directory").
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

arch/csky/boot/dts/include/dt-bindings

-../../../../../include/dt-bindings
\ No newline at end of file