pwm: pwm-tiehrpwm: Update DT binding document to use generic node name

Now that the node name has been changed from ehrpwm to pwm the document
should show this proper usage. Also change the unit address in the example
from 0 to the proper physical address value that should be used.

Signed-off-by: Franklin S Cooper Jr <fcooper@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>