drm/armada: Include <linux/of.h>

Include <linux/of.h> to get declarations of of_node_put() and
of_device_is_available(). No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sui Jingfeng <suijingfeng@loongson.cn>
Link: https://patchwork.freedesktop.org/patch/msgid/20230330073046.7150-2-tzimmermann@suse.de