powerpc/512x: clk: Include clk.h

This clock provider uses the consumer API, so include clk.h
explicitly.

Cc: Gerhard Sittig <gsi@denx.de>
Cc: Scott Wood <scottwood@freescale.com>
Acked-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>