ARM: tegra: dts: Add pwm label

PWM devices can be referenced in the DT by phandle and per-chip index.
In order for this to work properly, the PWM controller needs to have a
label attached to it.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Stephen Warren <swarren@nvidia.com>