clk: mmp: Remove clk.h include

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, so drop
it and add in slab.h to keep things compiling.

Cc: Chao Xie <chao.xie@marvell.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>