iio: pulse: add TI ECAP driver

Adds support for capturing PWM signals using the TI ECAP peripheral.
This driver supports triggered buffer capture of pulses on multiple
ECAP instances. In addition, the driver supports configurable polarity
of the signal to be captured.

Signed-off-by: Matt Porter <matt.porter@linaro.org>
pwm: enable TI PWMSS if the IIO tiecap driver is selected

The IIO TI ECAP driver depends on the TI PWMSS management
driver in this subsystem. Enable it when we select the IIO
TI ECAP driver.

Signed-off-by: Matt Porter <matt.porter@linaro.org>
iio: enable selection and build of pulse drivers

Add the pulse driver subdirectory when configuring and building
IIO.

Signed-off-by: Matt Porter <matt.porter@linaro.org>
ARM: dts: AM33XX: Add ecap interrupt properties

Add missing interrupt properties to the ecap0, ecap1, and ecap2
nodes.

Signed-off-by: Matt Porter <matt.porter@linaro.org>
ARM: dts: AM33XX: enable ecap2 on Beaglebone Black

Enables ecap2 and sets ecap/input mode on P9-28

Signed-off-by: Matt Porter <matt.porter@linaro.org>