UPSTREAM: pwm: sysfs: Add PWM capture support

UPSTREAM: pwm: sysfs: Add PWM capture support

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: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>
(cherry picked from commit 1a366fe9153f445e950a7a344932b7419aa83094)

Change-Id: I86326709c373630a9189232a1cf2804a1a6ed380
Signed-off-by: David Wu <david.wu@rock-chips.com>