arm64: dts: broadcom: bcmbca: bcm4908: Add DT for Zyxel EX3510-B

arm64: dts: broadcom: bcmbca: bcm4908: Add DT for Zyxel EX3510-B

Zyxel EX3510-B is a WiFi 6 capable home gateway (family) based on the
BCM4906 SoC, with 512MiB of RAM and 512MiB of NAND flash. WiFi support
consists of a BCM6710 and a BCM6715 attached to separate PCIe buses.

Add an initial devicetree for this system, with support for:
- Onboard UART (per base dtsi)
- USB (2.0 only; superspeed devices are treated as high-speed due to an
    unknown cause)
- Both buttons (rear reset, front WPS)
- Almost all LEDs:
  - Power (red/green)
  - Internet (red/green)
  - WAN (green)
  - LAN (green; anode is connected to GPIO 13 so currently
      nonfunctioning)
  - USB (green)
  - WPS button (red/green)
  - Absent in DT: There are 2.4GHz/5.0GHz WiFi status LEDs connected to
      the WiFi chips instead of the SoC.
- NAND flash
- Embedded Ethernet switch
- Factory-programmed Ethernet MAC address

WiFi cannot be enabled at this time due to Linux lacking drivers for
both the PCIe controllers and the PCIe WiFi peripherals.

Signed-off-by: Sam Edwards <CFSworks@gmail.com>
Link: https://lore.kernel.org/r/20241009215454.1449508-3-CFSworks@gmail.com
Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>