dts/riscv/openisa: add compatible strings for the RI5CY cores

This commits adds two new compatible strings:
* `openisa,ri5cy`
* `openisa,zero-ri5cy`

Adding these two new compats help identify the specific core defined by the
cpu node from the devicetree alone.

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