dt-bindings: gpio: gpio-mvebu: deprecate armadaxp-gpio

Commit 5f79c651 ("arm: mvebu: use global interrupts for GPIOs on
Armada XP") the marvell,armadaxp-gpio compatible obsolete.

The driver code still exists to handle the armadaxp behaviour but all
the in-tree boards use the marvell,armada-370-gpio.  Document the
marvell,armadaxp-gpio compatible as deprecated.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>