riscv: Ensure the kernel start address is correctly cast

riscv: Ensure the kernel start address is correctly cast

Cast the kernel start address to the target bit length.

This ensures that we calculate the initrd offset to a valid address for
the architecture.

Steps to reproduce the original problem (reported by Alex):
  Build U-Boot for the virt machine for riscv32. Then run it with

    $ qemu-system-riscv32 -M virt -kernel u-boot -nographic -initrd <a file>

  You can find the initrd address with

    U-Boot# fdt addr $fdtcontroladdr
    U-Boot# fdt ls /chosen

  Then take a peek at that address:

    U-Boot# md.b <addr>

  and you will see that there is nothing there without this patch. The
  reason is that the binary was loaded to a negative address.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Suggested-by: Alexander Graf <agraf@suse.de>
Reported-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>