dt-bindings: arm: update Armada CP110 system controller binding

It turns out that in the CP110 HW block present in Marvell Armada
7K/8K SoCs, gatable clock n°18 not only controls SD/MMC, but also the
GOP block. This commit updates the Device Tree binding for this piece
of hardware accordingly.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>