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. Change the unit address in the example
from 0 to the proper physical address value that should be used. Also
insure that the unit address matches to the reg property.

Signed-off-by: Franklin S Cooper Jr <fcooper@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>