uboot-imx

sandbox: Add a note about the growing state_info struct

This struct is getting larger and in some cases is being used for things
which would be better put into a driver. For example hwspinlock is not
used outside of sandbox_hwspinlock.c.

Add a note to encourage people to put things elsewhere.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>