net: dsa: lantiq_gswip: split into common and MMIO parts

net: dsa: lantiq_gswip: split into common and MMIO parts

Move all parts specific for the MMIO/SoC driver into a module of its own
to prepare for supporting MDIO-connected switch ICs.
Modify gswip_probe() functions by splitting it into a common function
gswip_probe_common() which covers allocating, initializing and registering
the DSA switch, while keeping transport-specific regmap initialization as
well as PHY firmware loading in the new MMIO/SoC-specific gswip_probe()
function.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Tested-by: Alexander Sverdlin <alexander.sverdlin@siemens.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://patch.msgid.link/dc7da5b65ec220ba8e9bc4bd04fe1ed7de046656.1762170107.git.daniel@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>