lib: utils/fdt/fdt_domain: Simplify region access permission check

The region access permission check in __fdt_parse_region() can be
simplified as masking SBI_DOMAIN_MEMREGION_{M,SU}_ACCESS_MASK is
enough.

While we are here, update the confusing comments to match the codes.

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