ARM: dts: mvebu: add missing interrupt to 98dx4251 switch

The 98dx4251 has 4 interrupts for the packet processor whereas the
98dx3236 and 98dx3336 only have 3.

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