sifive: handle sfence.vma errata

[mhorne]

Summary

CIP-1200 on the SiFive FU740 (HiFive Unmatched) states that an I-TLB
refill concurrent with an sfence.vma instruction may erroneously refill
stale mappings. The recommended workaround is to issue a global
sfence.vma in all cases.

Thus, add an errata variable and tweak the pmap functions to use the
global sfence in all cases.

The exact CPU identification IDs were verified against Linux.

Test Plan

Reference document: https://www.sifive.com/document-file/freedom-u740-c000-errata1

To be booted on Unmatched board; I have replaced everything except sfence_vma() with .unimp directive, to ensure that only the global fence is executed.