qemu-riscv64: new device (MR 3571)

Support launching postmarketOS in QEMU on riscv64 architecture. It is
mostly copied from device-qemu-aarch64 with some riscv64-specific
modifications.

Currently UIs using Xorg like Mate, XFCE4 and LXQT work fine, Wayland
UIs like weston, plasma-mobile etc didn't work in my testing.

Also using "pmbootstrap qemu --tablet" is needed to make mouse input
work, the defaults don't seem to work at the moment.

[ci:skip-build]: already built successfully in CI