The DT files for pwm were merged and converted to json.
The new reference is already at the maintainers file, so
just drop the obsoleted one.

Fixes: 56fb34d8 ("dt-bindings: mfd: Convert stm32 timers bindings to json-schema")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>