arm64: dts: rockchip: add missing #interrupt-cells to rk356x pcie2x1

This fixes the following issue:
  pcieport 0000:00:00.0: of_irq_parse_pci: failed with rc=-22

Signed-off-by: Jensen Huang <jensenhuang@friendlyarm.com>
Link: https://lore.kernel.org/r/20230113064457.7105-1-jensenhuang@friendlyarm.com
Signed-off-by: Heiko Stuebner <heiko@sntech.de>