ARM: vexpress: Fix wdt interrupt in ca15{-tc1,_a7} dts

As the wdt nodes have the gic as their interrupt-parent, their
interrupts property should be 3 cells in format described in the gic
devicetree binding document.

This patch fixes the interrupts property in the wdt nodes to be in the
correct format.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Pawel Moll <pawel.moll@arm.com>