Execute PL310_ERRATA_727915 only for related revisions

Part of PL310 erratum 727915 in pl310_wbinv_range() was
executed uncoditionally for all possible controllers'
revisions. This patch adds appropriate condition, since
extra operations are required only for revisions between
r2p0 and r3p0.

Submitted by: Marcin Wojtas <mw@semihalf.com>
Obtained from: Semihalf
Sponsored by: Stormshield
Reviewed by: meloun-miracle-cz
Differential revision: https://reviews.freebsd.org/D10221