dm: Split the simple malloc() implementation into its own file

The simple malloc() implementation is used when memory is tight. It provides
a simple buffer with an incrementing pointer.

At present the implementation is inside dlmalloc. Move it into its own file
so that it is easier to find.

Rather than using relocation as a signal that the full malloc() is
available, add a special GD_FLG_FULL_MALLOC_INIT flag. This signals that the
simple malloc() should no longer be used.

In some cases, such as SPL, even the code space used by the full malloc() is
wasteful. Add a CONFIG_SYS_MALLOC_SIMPLE option to provide only the simple
malloc. In this case the full malloc is not available at all. It saves about
1KB of code space and about 0.5KB of data on Thumb 2.

Acked-by: Tom Rini <trini@ti.com>
Signed-off-by: Simon Glass <sjg@chromium.org>