powerpc: dts: p2020: Add fsl/p2020si-pre.dtsi and fsl/p2020si-post.dtsi symlinks

P2020 DTS files in upstream Linux kernel use fsl/p2020si-pre.dtsi and
fsl/p2020si-post.dtsi include device tree files.

Add symlinks for these include device tree files into U-Boot powerpc
directory and points them to U-Boot inline device tree files p2020.dtsi and
p2020-post.dtsi.

This allows to use P2020 DTS files from upstream Linux kernel in U-Boot.

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