CRIS: stop checking for MTD_CONCAT

As MTD_CONCAT support is becoming an integral part of MTD core,
there is no need for it's special treatment. So stop checking for
MTD_CONCAT availability.

Acked by Jesper Nilsson <jesper.nilsson@axis.com> for merging this
via the MTD tree.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Acked-by: Stefan Roese <sr@denx.de>
Signed-off-by: Jesper Nilsson <jesper.nilsson@axis.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>