cmd: Add an indication of 32/64-bit to bdinfo

It is useful to know what mode U-Boot is running in. Add a message at the
end of the 'bdinfo' output.

Suggested-by: Mark Kettenis <kettenis@openbsd.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: change commit tag to 'cmd' as this is not x86 specific]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>