pwm: Constify OF match tables

A few drivers already annotate this properly. Make the same change for
all other OF supporting drivers.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Alexandre Pereira da Silva <aletes.xgr@gmail.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>