i2c: mlxcpld: Add callback to notify probing completion

Add notification to inform caller that driver probing has been
completed. It allows to user, invoked platform device registration for
"i2c-mlxcpld" driver, to be notified that bus adapter is available, and
thus some devices could be connected to this bus.

Signed-off-by: Vadim Pasternak <vadimp@nvidia.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>