dt-bindings: arm: atmel: describe SECUMOD usage as a GPIO controller

This patch describes the Security Module's usage as a GPIO
controller for its PIOBU pins. These pins have the special
property of maintaining their voltage during suspend-to-mem.

Signed-off-by: Andrei Stefanescu <andrei.stefanescu@microchip.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>