Add 3 READ ports when using the X-IF

[davideschiavone]

When using the X-IF, it would be good to support 3 read ports.

Why?

1. To support RVF or RVF_Zinf
2. Because while it is gonna be more expensive area-wise, the X-IF flavour is meant to be attached to a coprocessor, so the total area overhead would diminish.
3. To support a wider ISA extension use-cases

[StMiky]

We have use cases where support for R4-type instructions offloading would make CVE2 the first choice among the OpenHW CPUs. If X_NUM_RS is kept parametric, the area overhead of this feature should be negligible when it's not needed.