ARM: dts: mvebu: disable the rtc on 98dx3236 SoC

There is no rtc for the 98dx3236 and derivative SoCs. Disable the rtc
node inherited from the armada-370-xp base.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>