Add an spl_init() function that does basic init such that board_init_f() can
use simple malloc(), device tree and driver model. Each one is set up only
if enabled for SPL.

Note: We really should refactor SPL such that there is a single
board_init_f() and rename the existing weak board_init_f() functions
provided by boards, calling them from the single board_init_f().

Signed-off-by: Simon Glass <sjg@chromium.org>