ARM: socfpga: clk: Convert to clock framework

Use clock framework functions to fetch clock information now that there
is a clock driver for Arria10, instead of custom coded register parsing.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Chin Liang See <chin.liang.see@intel.com>
Cc: Dinh Nguyen <dinguyen@kernel.org>
Cc: Ley Foon Tan <ley.foon.tan@intel.com>