pwm: tegra: Add device tree support

Add auxdata to instantiate the PWFM controller from a device tree,
include the corresponding nodes in the dtsi files for Tegra 20 and
Tegra 30 and add binding documentation.

Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>