clk: mvebu: Remove clk.h include

Clock provider drivers generally shouldn't include clk.h because
it's the consumer API. Only include the header if necessary. The
clkdev.h include isn't used here either, so drop it and add in
slab.h to keep things compiling.

Cc: Boris Brezillon <boris.brezillon@free-electrons.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Gregory CLEMENT <gregory.clement@free-electrons.com>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Cc: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>