dts/riscv/efinix: add the `efinix,vexriscv-sapphire` compatible string

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

The VexRiscv core configuration is specific to the Efinix Sapphire SoC.

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