clk: socfpga: Remove clk.h and clkdev.h includes

Clock provider drivers generally shouldn't include clk.h because
it's the consumer API. Remove the include here because this is a
provider driver. The clkdev.h include isn't used either, remove
it and add in slab.h to make sure things keep compiling.

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