dt-bindings: media: remove nokia,n900-ir as pwm-ir-tx is compatible

The generic pwm-ir-tx driver works for the Nokia n900, so nokia,n900-ir
can be removed.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Lee Jones <lee@kernel.org>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>