pwm: Add missing static storage class specifiers in core.c file
Fixes the following sparse warnings:
drivers/pwm/core.c:152:6: warning:
symbol 'of_pwmchip_add' was not declared. Should it be static?
drivers/pwm/core.c:165:6: warning:
symbol 'of_pwmchip_remove' was not declared. Should it be static?