clk: socfpga: split clk code

Move the different kinds of clocks into their own files. The reason is to aid
readability of the code. This also goes along with the other SoC-specific
clock drivers.

The split introduces new structs for the three types of clocks and uses them.
Other changes are not done to the code.

Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Dinh Nguyen <dinguyen@altera.com>