dts: set the `riscv,isa` property for virt-based targets

This commit makes the devicetrees of the targets that are based on the QEMU
`virt` machine more consistent with the rest of the RISC-V targets in
Zephyr by:
* adding the `riscv,isa` property
* adding a compatible string which uniquely identifies the `virt` core

Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>