clk: mmp: add missing header dependencies

We get 1 warning when building kernel with W=1:
drivers/clk/mmp/clk-mmp2.c:75:13: warning: no previous prototype for 'mmp2_clk_init' [-Wmissing-prototypes]

In fact, this function is declared in linux/clk/mmp.h,
so this patch add missing header dependencies.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>