mediatek: mt7622: enable CONFIG_MTD_VIRT_CONCAT

mediatek: mt7622: enable CONFIG_MTD_VIRT_CONCAT

Some devices have a ton of unused space found in other partitions that
can be used to expand rootfs_data.

In particular, the D-Link DIR-X3260 device can squeeze another ~24MiB
from its backup partition, with a few tricks, allowing for ~48MiB of
usable space rather than ~24MiB by itself.

As of v25.12.1, with the official release build config, this gives us
~35MiB of rootfs_data, where without it there would only be ~11MiB.

Enable this config to allow this function.

Signed-off-by: Han Sol Jin <jhs@jinhansol.com>
Link: https://github.com/openwrt/openwrt/pull/22555
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>