MN10300: AM34: The current cacheflush routines operate by controlling tag regs

The current cache flush and invalidate routines operate by controlling the
cache tag registers.  Rename the files and add config items to select them.

This makes it easier to support the use of other cache flush methods instead,
such as the use of AM34's area purge registers, if available.

Signed-off-by: David Howells <dhowells@redhat.com>