clk: spear: Move prototype to accessible header

Fixes the following W=1 kernel build warning(s):

 drivers/clk/spear/spear6xx_clock.c:116:13: warning: no previous prototype for function 'spear6xx_clk_init' [-Wmissing-prototypes]

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>