qemu-riscv64: new device

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.

Maybe should go through testing? Not sure how we want to do this.

Also not sure, @Minecrell @drebrez would you be also willing to maintain this QEMU package in addition to x86_64 & aarch64? The device package is mostly copy-paste from aarch64 one iirc, plus extlinux.conf

For testing in QEMU you need to use pmbootstrap!2219 (closed) ~~https://lists.sr.ht/~postmarketos/pmbootstrap-devel/patches/36463~~ https://lists.sr.ht/~postmarketos/pmbootstrap-devel/patches/36934