dt-bindings: pwm: sti: Update DT bindings for capture support

In order to support capture of PWM signals a new clock as well as an
interrupt line are required. Furthermore a new property is introduced
that allows the number of capture channels to be specified. Typically
there will be less capture channels than output channels, since all
channels have the latter capability, but only some have support for
capture.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>