Allow a user to read PWM capture results from sysfs. To start a capture
and read the result, simply read the file:

  $ cat $PWMCHIP/capture

The output format is "<period> <duty cycle>".
Signed-off-by: NLee Jones <lee.jones@linaro.org>
Signed-off-by: NThierry Reding <thierry.reding@gmail.com>