board: sifive: Fix -Wint-to-pointer-cast warning

The following warning is seen in unleashed.c in a 32-bit build:

  warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]

Cast with uintptr_t.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>