powerpc/85xx: Rework P2020RDB device tree

Utilize new split between board & SoC, and new SoC device trees split
into pre & post utilizing 'template' includes for SoC IP blocks.

Other changes include:

* Moved to specifying interrupt-parent for mpic at root
* Moved to 4-cell mpic interrupt cells to support MPIC timers
* Reworked PCIe nodes to allow supportin IRQs for controller (errors) and
  moved PCI device IRQs down to virtual bridge level
* Updated spi node to new espi binding specification
* Renamed 'sdhci' node to 'sdhc'
* Changed GPIO compatiable from 'fsl,mpc8572-gpio' to 'fsl,pq3-gpio' as the
 'mpc8572' compatiable is to deal with a 'mpc8572' specific to an erratum
* Fixed wrong reg offsets for mdio nodes associated with etsec2 &
* etsec3
* Dropping "fsl,p2020-IP..." from compatibles for standard blocks

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>