This function will be used by both SPL and U-Boot proper. So move it into
a common place. Also change the #ifdef so that the early malloc() area is
not set up in SPL if CONFIG_SYS_SPL_MALLOC_START is defined. In that case
it would never actually be used, and just chews up stack space.

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