Add Support for J721E Common Processor board support.
The EVM architecture is as follows:

+------------------------------------------------------+
|   +-------------------------------------------+      |
|   |                                           |      |
|   |        Add-on Card 1 Options              |      |
|   |                                           |      |
|   +-------------------------------------------+      |
|                                                      |
|                                                      |
|                     +-------------------+            |
|                     |                   |            |
|                     |   SOM             |            |
|  +--------------+   |                   |            |
|  |              |   |                   |            |
|  |  Add-on      |   +-------------------+            |
|  |  Card 2      |                                    |    Power Supply
|  |  Options     |                                    |    |
|  |              |                                    |    |
|  +--------------+                                    | <---
+------------------------------------------------------+
                                Common Processor Board

Common Processor board is the baseboard that has most of the actual
connectors, power supply etc. A SOM (System on Module) is plugged on
to the common processor board and this contains the SoC, PMIC, DDR and
basic high speed components necessary for functionality. Add-n card
options add further functionality (such as additional Audio, Display,
networking options).

Note:
A) The minimum configuration required to boot up the board is System On
   Module(SOM) + Common Processor Board.
B) Since there is just a single SOM and Common Processor Board, we are
   maintaining common processor board as the base dts and SOM as the dtsi
   that we include. In the future as more SOM's appear, we should move
   common processor board as a dtsi and include configurations as dts.
C) All daughter cards beyond the basic boards shall be maintained as
   overlays.
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>