configs: visionfive2: enable bootstage configs

Enable BOOTSTAGE configuration and its command for visionfive2 board.
The feature can be useful for analyzing the elapsed time between boot
stages.

TODO: define / reserve memory region for boot stage stash

StarFive # bootstage report
Timer summary in microseconds (10 records):
       Mark    Elapsed  Stage
          0          0  reset
  3,139,338  3,139,338  board_init_f
  3,176,753     37,415  board_init_r
  4,036,111    859,358  eth_common_init
  4,101,599     65,488  eth_initialize
  4,105,799      4,200  main_loop
  4,145,207     39,408  usb_start
  5,440,963  1,295,756  cli_loop

Accumulated time:
                10,093  dm_f
                15,867  dm_r

Signed-off-by: Chanho Park <chanho61.park@samsung.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>