include: sbi_utils: Introduce an helper to get fdt base address

This simply adds an helper to get fdt address which is more explicit than
sbi_scratch_thishart_arg1_ptr.

Signed-off-by: Alexandre Ghiti <alexandre.ghiti@canonical.com>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>