riscv: sifive: unleashed: Switch to use binman to generate u-boot.itb

At present SiFive Unleashed board uses the Makefile to create the FIT,
using USE_SPL_FIT_GENERATOR, which is deprecated as per the Makefile
warning. Update to use binman instead.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>