clk: mxs: Include clk mxs header file

clk: mxs: Include clk mxs header file

Fix the following sparse warnings:

drivers/clk/mxs/clk-imx28.c:72:5: warning: symbol 'mxs_saif_clkmux_select' was not declared. Should it be static?
drivers/clk/mxs/clk-imx28.c:156:12: warning: symbol 'mx28_clocks_init' was not declared. Should it be static?

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Mike Turquette <mturquette@linaro.org>
[mturquette@linaro.org: fixed $SUBJECT line]