rockchip: Fix SPL console output when ROCKCHIP_SPL_BACK_TO_BROM is enabled

Move back_to_bootrom() call later in SPL init so that the console is
initialized and printouts happen.

Currently when ROCKCHIP_SPL_BACK_TO_BROM is enabled there is no console
output from the SPL init stages.

I wasn't sure exactly where this should happen, so if we are set to do
run spl_board_init, then go back to bootrom there after
preloader_console_init(). Otherwise fall back to old behavior of doing
it in board_init_f.

Signed-off-by: Sandy Patterson <apatterson@sightlogix.com>
Acked-by: Ziyuan Xu <xzy.xu@rock-chips.com>
Acked-by: Simon Glass <sjg@chromium.org>