usr.sbin/i2cscan: support scans using 1 byte reads

Add '-r' to probe devices using 1 byte reads instead
of 0 byte writes as 0 byte writes aren't supported on
the am335x nor the dm37xx.

Submitted Upstream: http://gnats.netbsd.org/47977

Change-Id: I11bd030e93d2eb8de02c67f263fe5da8950621f3