pwm: sti: Initialise PWM capture device data

Each PWM capture device is allocated a structure to hold its own state.
During a capture the device may be partaking in one of 3 phases. Initial
(rising) phase change, a subsequent (falling) phase change indicating
end of the duty-cycle phase and finally a final (rising) phase change
indicating the end of the period. The timer value snapshot each event is
held in a variable of the same name, and the phase number (0, 1, 2) is
contained in the index variable. Other device specific information, such
as GPIO pin, the IRQ wait queue and locking is also contained in the
structure. This patch initialises this structure for each of the
available devices.

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