arm64: dts: marvell: add a reference to the sysctrl syscon in the ppv2 node

The network driver on Marvell SoC (7k/8k) needs to access some registers
in the system controller to configure its ports at runtime. This patch
adds a phandle reference to the syscon system controller node in the
ppv2 node.

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Tested-by: Marcin Wojtas <mw@semihalf.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>