pwm: pwm-ti*: Get the clock from the PWMSS parent when using old bindings

When using the old eCAP and ePWM bindings for AM335x and AM437x the clock
can be retrieved from the PWMSS parent. Newer bindings will insure that
this clock is provided via device tree.

Therefore, update this driver to support the newer and older bindings. In
the case of the older binding being used give a warning.

Signed-off-by: Franklin S Cooper Jr <fcooper@ti.com>
[thierry.reding@gmail.com: rewrite slightly for readability]
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>