Fix warnings with clang like:

arch/arm/mach-spear/spear6xx.c:365:13: warning: no previous prototype for function 'spear6xx_map_io' [-Wmissing-prototypes]

by making few definitions static.
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>