clk: at91: Include clk.h and slab.h

This clock provider uses the consumer API, so include clk.h
explicitly. Also include slab.h instead of relying on clkdev.h
for it.

Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>