Proposal: Add dependency to Zicsr

[a4lg]

Proposal

I propose to add a note that the Zpn extension depends on the Zicsr extension.

Background

All legal combinations of RV32P/RV64P contain the Zpn extension, which contains RDOV and CLROV pseudoinstructions. They access vxsat CSR (URW, 0x9) and require the Zicsr extension.

This is basically the same issue report as riscvarchive/riscv-code-size-reduction#179 that is resolved by adding an extension dependency (Zcmt → Zicsr).

Notes

Although that OV flag is updated by some instructions in Zpsfoperand, I'm not sure whether Zpsfoperand should depend on the Zicsr (I think that Zpsfoperand → Zicsr is not necessary) because:

1. Such access to OV flag is write-only
2. Zpsfoperand does not contain instructions/pseudoinstructions that access OV flag via CSR.

[Kevin-Andes]

This is a very good suggestion and we should add dependency to Zicsr for Zpn.