rockchip: rk3399: make spl_board_init board-specific

The later-stage spl_board_init (as opposed to board_init_f) should set
up board-specific details: these differ between the EVB-RK3399 and the
RK3399-Q7 (Puma).

This moves spl_board_init back into the individual boards and removes
the unneeded functionality from Puma.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>