powerpc: dts: p2020: Add serial0 and serial1 via pq3-duart-0.dtsi

Import pq3-duart-0.dtsi device tree include file from upstream Linux kernel
for P2020. This allows U-Boot to use P2020 device tree files from upstream
Linux kernel which reference serial0 or serial1 devices.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>