riscv: ax25-ae350: Cast addr with uintptr_t

addr was delcared as fdt_addr_t which is now a 64-bit address.
In a 32-bit build, this causes the following warning seen when
building ax25-ae350.c:

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

Cast addr with uintptr_t.

Signed-off-by: Bin Meng <bin.meng@windriver.com>