powerpc/85xx: P1020 DTS : re-organize dts files

Creates P1020si.dtsi, containing information for the P1020 SoC. Modifies dts
files for P1020 based systems to use dtsi file

Signed-off-by: Prabhakar Kushwaha <prabhakar@freescale.com>
Acked-by: Grant Likely <grant.likelY@secretlab.ca>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>