doc: rockchip: Update build instruction for rk3308

After commit d8765e2422cd ("Enable building of u-boot.itb
on Rockchip platform"), u-boot.itb will automatically
generated by "make all" command, manually command
"make u-boot.itb" is no longer needed.

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
1 file changed