rockchip: puma-rk3399: update board_init()

The original initialisation code for board_init() was largely lifted
from the code on the EVB.  However, the RK3399-Q7 can do with a much
more concise init sequence.

This cleans up the board_init() by updating it to the essentials for
the RK3399-Q7 and getting rid of the accumulated cruft.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>