arm: Show early-malloc() usage in bdinfo

This is useful information to show how close we are to the limit. At present
it is only available by enabling DEBUG in board_r.c.

Make it available with the 'bdinfo' command also.

Note that this affects ARM only. The bdinfo command is different for each
architecture. Rather than duplicating the code it would be better to
refactor it (as was done with global_data).

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