arm64: dts: add description for solidrun imx8mp hummingboard variants

arm64: dts: add description for solidrun imx8mp hummingboard variants

Add descriptions for the SolidRun i.MX8M Plus System on Module based
HummingBoard product-line. They share a common designed based on the
"Pulse" version, defined by various assembly options.

The HummingBoard Pulse features:
- 2x RJ45 Ethernet
- 2x USB-3.0 Type A
- HDMI connector
- mini-HDMI connector
- microSD connector
- mini-PCI-E connector with SIM slot supporting USB-2.0/3.0 interfaces
- M.2 connector with SIM slot supporting USB-2.0/3.0 interfaces
- MIPI-CSI Camera Connector (not described without specific camera)
- 3.5mm Analog Stereo Out / Microphone In Headphone Jack
- RTC with backup battery

The variants Mate and Ripple are reduced versions of Pulse.

The HummingBoard Pro extends Pulse with PCI-E on M.2 connector.

Signed-off-by: Josua Mayer <josua@solid-run.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>