Could I call the ARM-platform especially "Raspberry Pi" as "Open Hardware"? Exactly one answer: No.

If you want to build the ARM processor that drives it, the design of the CPU is not available and you are going to have to pay ARM holdings to get it. No doubt under NDA (Non-Disclosure Agreement) so you don't share it with anyone else.

If you want to build the chip that the ARM processor lives in. The "System On a Chip" or SOC, you are going to have to talk to the company Broadcom. I don't think they are willing to share the design of the GPU with you.

In short, there is nothing "open" about the hardware-platform "Raspberry Pi". The FSF have got also an article about that, besides that point there are already enough restrictions throughout bootloader and basic software-implementations: https://www.fsf.org/resources/hw/single-board-computers #Raspberry-Pi #Not-Free-and-Libre