clk: socfpga: allow for multiple parents on Arria10 periph clocks

There are some Arria10 clocks of type "altr,socfpga-a10-perip-clk" that can
have multiple parents. Fix up the __socfpga_periph_init() to call
of_clk_parent_fill() that will return the appropriate number of parents.

Also, update __socfpga_gate_init() to call of_clk_parent_fill() helper
function.

Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>