arm64: dts: socfpga: add missing interrupt-parent

The PMU node has no working interrupt, as shown by this dtc warning:

arch/arm64/boot/dts/altera/socfpga_stratix10_socdk.dtb: Warning (interrupts_property): Missing interrupt-parent for /pmu

This adds an interrupt-parent property so we can correct parse
that interrupt number.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Dinh Nguyen <dinguyen@kernel.org>
Signed-off-by: Olof Johansson <olof@lixom.net>