clk: hisilicon: Remove CLK_IS_ROOT

This flag is a no-op now. Remove usage of the flag.

Tested-by: Leo Yan <leo.yan@linaro.org>
Cc: Bintian Wang <bintian.wang@huawei.com>
Cc: Zhangfei Gao <zhangfei.gao@linaro.org>
Cc: Haojian Zhuang <haojian.zhuang@gmail.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>