ARM: Dove: fix bad properties of the si5351 clkout2 used by Cubox audio

As defined in the DT, clkout2 is not allowed to change the pll inside
si5351.

This patch extends the properties of clkout2 so that it works as the
external clock of the audio device in the Cubox.

Also, as the second si5351 clock is not used in the Cubox, its
definition is removed.

Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Acked-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>