clk: socfpga: use clk_hw_register for a5/c5

As recommended by Stephen Boyd, convert the cyclone5/arria5 clock driver
to use the clk_hw registration method.

Suggested-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>
Link: https://lore.kernel.org/r/20210302214151.1333447-1-dinguyen@kernel.org
Signed-off-by: Stephen Boyd <sboyd@kernel.org>