clk: si570: Include clk.h

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

Cc: Guenter Roeck <linux@roeck-us.net>
Acked-by: Sören Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>