clk: st: make use of of_clk_parent_fill helper function

Use of_clk_parent_fill to fill in the parent clock names' array.

Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>
Tested-by Gabriel Fernandez <gabriel.fernandez@st.com>
Cc: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>