include: fdt/fdt_helper: Change fdt_get_address() to return root.next_arg1

In sbi_domain_finalize(), when locating the coldboot hart's domain,
the coldboot hart's scratch->arg1 will be overwritten by the domain
configuration. However scratch->arg1 holds the FDT address of the
coldboot hart, and is still being accessed by fdt_get_address() in
later boot process. scratch->arg1 could then contain completely
garbage and lead to a crash.

To fix this, we change fdt_get_address() to return root domain's
next_arg1 as the FDT pointer.

Resolves: https://github.com/riscv-software-src/opensbi/issues/281
Fixes: b1678af2

 ("lib: sbi: Add initial domain support")
Reported-by: Marouene Boubakri <marouene.boubakri@nxp.com>
Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Anup Patel <anup@brainfault.org>