clk: hi6220: add debug APB clock

The debug APB clock is absent in hi6220 driver, so this patch is to add
support for it.

Signed-off-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>