spl: Tidy up SPL/TPL malloc sizes

The current help talks about relocation which doesn't apply to SPL and
TPL. Update it to avoid confusion.

Also make the TPL size default to the same as the SPL size, since this is
more likely to be a useful value than the one used by U-Boot proper, which
may be quite a bit larger.

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