UPSTREAM: pwm: Add PWM capture support

UPSTREAM: pwm: Add PWM capture support

Supply a PWM capture callback op in order to pass back information
obtained by running analysis on a PWM signal. This would normally (at
least during testing) be called from the sysfs routines with a view to
printing out PWM capture data which has been encoded into a string.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
[thierry.reding@gmail.com: make capture data unsigned int for symmetry]
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>
(cherry picked from commit 3a3d1a4e32ab47323d7b8c8b7631a8d36a3098b2)

Change-Id: Iee3acf2eb02c5282d586f4e7f1745031e17cc383
Signed-off-by: David Wu <david.wu@rock-chips.com>