spl: Add spl_early_init()

At present malloc_base/_limit/_ptr are not initialised in spl_init() when
we call spl_init() in board_init_f(). This is due to a recent change aimed
at avoiding overwriting the malloc area set up on some boards by
spl_relocate_stack_gd().

However if CONFIG_SPL_STACK_R_MALLOC_SIMPLE_LEN is not defined, we now
skip setting up the memory area in spl_init() which is obviously wrong.

To fix this, add a new function spl_early_init() which can be called in
board_init_f().

Fixes: b3d2861e (spl: Remove overwrite of relocated malloc limit)
Signed-off-by: Eddie Cai <eddie.cai.linux@gmail.com>
Rewrote spl_{,early_}init() to avoid duplicate code:
Rewrite/expand commit message:
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Eddie Cai <eddie.cai.linux@gmail.com>