dts/riscv/litex: add `litex,vexriscv-standard` compatible string

This commit adds the `litex,vexriscv-standard` compatible string. This
helps identify the core type from the final devicetree alone.

The VexRiscv core version is defined in this repository:
https://github.com/litex-hub/zephyr-on-litex-vexriscv

.

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