lib: utils/fdt: Don't use sbi_string functions

When SBI is built by external firmware, we need to use their functions,
defined in libfdt_env.h.

Just like 2cfd2fc9



Signed-off-by: Daniel Schaefer <daniel.schaefer@hpe.com>
Reviewed-by: Abner Chang <abner.chang@hpe.com>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>