arm: dts: Add initial support for AM68 Starter Kit System on Module

arm: dts: Add initial support for AM68 Starter Kit System on Module

AM68 Starter Kit (SK) is a low cost, small form factor board designed
for TI’s AM68 SoC. TI’s AM68 SoC comprises of dual core A72, high
performance vision accelerators, hardware accelerators, latest C71x
DSP, high bandwidth real-time IPs for capture and display. The SoC is
power optimized to provide best in class performance for industrial
applications.

        AM68 SK supports the following interfaces:
        * 16 GB LPDDR4 RAM
        * x1 Gigabit Ethernet interface
        * x1 USB 3.1 Type-C port
        * x2 USB 3.1 Type-A ports
        * x1 PCIe M.2 M Key
        * 512 Mbit OSPI flash
        * x2 CSI2 Camera interface (RPi and TI Camera connector)
        * 40-pin Raspberry Pi GPIO header

SK's System on Module (SoM) contains the SoC, PMIC, DDR and OSPI flash.
Therefore, add support for the components present on the SoM.

Schematics: https://www.ti.com/lit/zip/SPRR463
TRM: http://www.ti.com/lit/pdf/spruj28

Signed-off-by: Sinthu Raja <sinthu.raja@ti.com>