dt-bindings: usb: atmel: Mark EP child node as deprecated

There is no need to describe the end point in the deice tree. These
properties won't be use anymore, so mark them as deprecated to keep
the old device tree documented.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>