cmd/sbi: add missing SBI information

Let the sbi command display:

* machine vendor ID
* machine architecture ID
* machine implementation ID

With this patch the output for the HiFive Unmatched looks like

    => sbi
    SBI 0.3
    OpenSBI 0.9
    Machine:
      Vendor ID 489
      Architecture ID 8000000000000007
      Implementation ID 20181004
    Extensions:
      sbi_set_timer
      sbi_console_putchar
      sbi_console_getchar
      sbi_clear_ipi
      sbi_send_ipi
      sbi_remote_fence_i
      sbi_remote_sfence_vma
      sbi_remote_sfence_vma_asid
      sbi_shutdown
      SBI Base Functionality
      Timer Extension
      IPI Extension
      RFENCE Extension
      Hart State Management Extension
      System Reset Extension

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Sean Anderson <seanga2@gmail.com>