pwm: pwm-tiehrpwm: Update device-tree binding document

Update binding document of pwm-tiehrpwm to reflect the usage of similar
modules in da850 and am3xx SOCs.

Signed-off-by: Philip Avinash <avinashphilip@ti.com>
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: Rob Landley <rob@landley.net>
Acked-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>