pwm: spear: Staticize spear_pwm_config()

spear_pwm_config() is not referenced outside of this file, make it static.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>