Include zynq clk header where init function is declared.

It removes this sparse warning:
drivers/clk/clk-zynq.c:373:13: warning: symbol
'xilinx_zynq_clocks_init' was not declared. Should it be static?
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>