rockchip: tpl: allow to call board/SoC-specific code before DRAM init

rockchip: tpl: allow to call board/SoC-specific code before DRAM init

This defines a weak tpl_board_init function that can be used for running
board/SoC-specific code before the DRAM init happens, similarly to
spl_board_init() for SPL.

Reviewed-by: Paul Kocialkowski <paulk@sys-base.io>
Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>