serial: max310x: Document clock setup

As a kernel newcomer, I got bitten by lack of examples on this front. I
had troubles figuring out where these clocks could be defined ("/clocks"
is where the generic infrastructure expects them).

One should also ensure that a unique name is used. Generic names such as
"osc" tend to be already used by some board-wide clock crystals.

Signed-off-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>