arm: sam9_l9260_defconfig: prepare to UBI config change

We are going to kill the CONFIG_MTD_UBI_BEB_RESERVE configuration option soon
and use the CONFIG_MTD_UBI_BEB_LIMIT instead. In order to do this smoothly,
we now introduce the new configuration option to sam9_l9260_defconfig, and
will kill the old one after the corresponding UBI changes are done.

Signed-off-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>