rockchip: add boot-mode support for rk3288, rk3036

rockchip platform have a protocol to pass the the kernel reboot mode to bootloader
by some special registers when system reboot. In bootloader we should read it and take action.

We can only setup boot_mode in board_late_init becasue "setenv" need env setuped.
So add CONFIG_BOARD_LATE_INIT to common header and use a entry "rk_board_late_init"
to replace "board_late_init" in board file.

Signed-off-by: Jacob Chen <jacob2.chen@rock-chips.com>
Acked-by: Simon Glass <sjg@chromium.org>