m32r: don't offer CONFIG_ISA

As far as I know no M32R hardware actually has ISA slots.

And ISA drivers don't compile on M32R.

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Hirokazu Takata <takata@linux-m32r.org>