ts72xx: use MTDPART_OFS_RETAIN for mtd partitioning

Instead of specifying a callback for dynamic partitioning, use
MTDPART_OFS_RETAIN for reserving a place near the end of flash for
RedBoot.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Cc: Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ryan Mallon <ryan@bluewatersys.com>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>