Change PULP_ZFINX ISA to the RISC-V ZFINX ISA

[davideschiavone]

we should move from the custom PULP ZFINX ISA to the RISC-V ZFINX ISA

This implies:

• to change the documentation
• to change the Parameter name
• to change the RTL - the RTL must be logically equivalent to the core when the Zfinx parameter is set to 0

Task assigned to @tariqkurd-repo