rockchip: rk3288-tinker: Change to use FIT

rockchip: rk3288-tinker: Change to use FIT

Change to use FIT and FIT_SIGNATURE when loading U-Boot proper in SPL to
allow checksum validation and fallback loading of FIT from a different
mmc device.

Checksum validation of FIT adds around 140 ms to boot time:

Before:

       Mark    Elapsed  Stage
          0          0  reset
         13         13  TPL
      6,957      6,944  end tpl
     25,102     18,145  SPL
    131,932    106,830  end phase
    132,137        205  board_init_f
    444,277    312,140  board_init_r
  1,404,987    960,710  eth_common_init
  1,519,110    114,123  eth_initialize
  1,524,734      5,624  main_loop
  1,525,452        718  cli_loop

After:

       Mark    Elapsed  Stage
          0          0  reset
         13         13  TPL
      6,957      6,944  end tpl
     35,744     28,787  SPL
    271,220    235,476  end phase
    271,420        200  board_init_f
    588,474    317,054  board_init_r
  1,548,950    960,476  eth_common_init
  1,663,105    114,155  eth_initialize
  1,668,734      5,629  main_loop
  1,669,417        683  cli_loop

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>