powerpc: dts: p2020: Remove duplicate pic@40000 node

DT node pic@40000 is defined explicitly in p2020-post.dtsi file and also
transitionally via include file pq3-mpic.dtsi. Remove duplicate definition
from p2020-post.dtsi.

No change in final DTB file.

Signed-off-by: Pali Rohár <pali@kernel.org>