powerpc/fsl-booke: Add initial silicon device tree files for B4860 and B4420

B4860 and B4420 are similar that share some commonalities

* common features have been added in b4si-pre.dtsi and b4si-post.dtsi
* differences are added in respective silicon files of B4860 and B4420

There are several things missing from the device trees of B4860 and B4420:

* DPAA related nodes (Qman, Bman, Fman, Rman)
* DSP related nodes/information
* serdes, sfp(security fuse processor), thermal,
  gpio, maple, cpri, quad timers nodes

Signed-off-by: Shaveta Leekha <shaveta@freescale.com>
Signed-off-by: Zhao Chenhui <chenhui.zhao@freescale.com>
Signed-off-by: Li Yang <leoli@freescale.com>
Signed-off-by: Tang Yuantian <Yuantian.Tang@freescale.com>
Signed-off-by: Varun Sethi <Varun.Sethi@freescale.com>
Signed-off-by: Minghuan Lian <Minghuan.Lian@freescale.com>
Signed-off-by: Ramneek Mehresh <ramneek.mehresh@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: Vakul Garg <vakul@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>