arm64: dts: add description for solidrun am642 som and evaluation board

Add description for the SolidRun AM642 SoM, and HummingBoard-T
evaluation board.

The SoM features:
- 1x cpsw ethernet with phy
- 2x pru ethernet with phy
- eMMC
- spi flash (assembly option)

Additionally microSD and usb-2.0 otg are included in the SoM
description as they are supported boot sources for the SOC boot-rom.

The Carrier provides:
- 3x RJ45 connector
- 2x M.2 connector
- USB-2.0 Hub
- USB-A Connector
- LEDs
- 2x CAN transceiver
- 1x RS485 transceiver
- sensors

The M.2 connectors support either USB-3.1 or PCI-E depending on status
of a mux. By default the mux is switched off.

Signed-off-by: Josua Mayer <josua@solid-run.com>
Link: https://lore.kernel.org/r/20240219-add-am64-som-v7-3-0e6e95b0a05d@solid-run.com
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>