clk: keystone: 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>
Acked-by: Santosh Shilimkar <ssantosh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>