net: dsa: lantiq_gswip: move to dedicated folder

Move the lantiq_gswip driver to its own folder and update
MAINTAINERS file accordingly.
This is done ahead of extending the driver to support the MaxLinear
GSW1xx series of standalone switch ICs, which includes adding a bunch
of files.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Hauke Mehrtens <hauke@hauke-m.de>
Link: https://patch.msgid.link/a5923dee9a174501b284dc473bdec9dd89c68de1.1756520811.git.daniel@makrotopia.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>