generic: fu740: add workaround for CIP-1200 errata

Instruction TLB can fail to respect a non-global SFENCE thus we need to
flush the TLB using SFENCE.VMA x0, x0

See full description of CIP-1200 in Errata_FU740-C000_20210205 from
https://www.sifive.com/boards/hifive-unmatched



Signed-off-by: David Abdurachmanov <david.abdurachmanov@sifive.com>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>