arm64: dts: marvell: add RTC description for Armada 7K/8K

This RTC IP is found in the CP110 master and slave which are part of the
Armada 8K SoCs and of the subset family the Armada 7K.

There is one RTC in each CP but the RTC requires an external
oscillator. However on the Armada 80x0, the RTC clock in CP master is not
connected (by package) to the oscillator. So this one is disabled for the
Armada 8020 and the Armada 8040.

As the RTC clock in CP slave is connected to the oscillator this one is
let enabled. and will be used on these SoCs (80x0).

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>