doc: devicetree: bindings: fix spelling mistake

Fix typo: "triger" --> "trigger"

Acked-by: Guo Ren <guoren@kernel.org>
Signed-off-by: Flavio Suligoi <f.suligoi@asem.it>
Link: https://lore.kernel.org/r/20200615075835.15202-1-f.suligoi@asem.it


Signed-off-by: Rob Herring <robh@kernel.org>