pwm: tiecap: Staticize non-exported symbols

Both ecap_pwm_save_context() and ecap_pwm_restore_context() are only used in
this file, make them static.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>