dt-bindings: Add header file of hi6220 clock driver

Add the header file "hi6220-clock.h" used by both
hi6220 clock driver and hi6220 device tree file.

Suggested-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Bintian Wang <bintian.wang@huawei.com>
Tested-by: Will Deacon <will.deacon@arm.com>
Tested-by: Tyler Baker <tyler.baker@linaro.org>
Tested-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Michael Turquette <mturquette@linaro.org>