fdt_support: Add fdt_delete_disabled_nodes() and use in Turris MOX

Move Turris MOX specific remove_disabled_nodes() to fdt_support with
name fdt_delete_disabled_nodes(), so that others can potentially use it.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>