i2c: mux: mlxcpld: Add callback to notify mux creation completion

Add notification to inform caller that mux objects array has been
created. It allows to user, invoked platform device registration for
"i2c-mux-mlxcpld" driver, to be notified that mux infrastructure is
available, and thus some devices could be connected to this
infrastructure.

Signed-off-by: Vadim Pasternak <vadimp@nvidia.com>
Acked-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Wolfram Sang <wsa@kernel.org>