Add support for zephyr qemu_riscv64

[casaroli]

Description

We added some preliminary support for qemu_riscv64 board but it is still not built and tested by the CI

Tasks

• Add storage_partition (.overlay)
• Add .config if necessary
• Add the build to the CI
• Add docs/boards/zephyr/qemu_riscv64.md
• Bonus 1: run zephyr mini / demo samples in the CI (test like we do with native_sim, after all this is qemu)
• Bonus 2: run zephyr system tests in the CI (after all this is qemu)
• Bonus 3: can we build a simple CI that can run mini, demo and system tests on "any" zephyr qemu we like? To make it easier to add new qemu targets.