Move mvswitch 88E6060 driver to the ath25

Move mvswitch 88E6060 driver to the ath25

ath25 requires a 88E6060 driver to support boards such as Fonera 2.0g
(FON2202). The swconfig based mvswitch driver has not yet been ported to
the 5.10 kernel as the only user is the ath25 target while all other
targets have been switched to the upstream DSA implementation.

Switching ath25 to the DSA implementation is a complex task, since we
need either per-board platform data or DTS support. ath25 lacks both of
them and builds only a single generic image. So we need to keep the
swconfig driver implementation to easly and quickly port ath25 to the
5.10 kernel.

Since porting the mvswitch driver to 5.10 as a generic driver is not an
option, and since the ath25 is its only user, make mvswitch a target
specific driver to be able to port it to the 5.10 kernel as part of the
kernel version update of the target. This will allow us quickly migrate
to the next kernel version and not delay the next firmware release.

Suggested-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>