docs: domain_support: Update the DT example

commit 3e2f573e

 ("lib: utils: Disallow non-root domains from adding M-mode regions")
added access permission check in __fdt_parse_region(). With the
existing DT example in the doc OpenSBI won't boot anymore.

Let's update the DT example so that it can work out of the box.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Anup Patel <anup@brainfault.org>