Building with clang, W=1, CONFIG_PM=n and CONFIG_OF_GPIO=n leads to
warning about unused ti_sn_pwm_pin_request() and
ti_sn_pwm_pin_release(). Fix by annotating them with __maybe_unused.
See also commit 6863f5643dd7 ("kbuild: allow Clang to find unused static
inline functions for W=1 build").