staging: ccg: Add a note about compatibility issues.

The first item on the todo list is a new user interface. Put this
information into Kconfig's help entry to people are not too confusing
once an user API changes which does not happen in kernel otherwise.

Cc: devel@driverdev.osuosl.org
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>