net: dsa: add driver for MaxLinear GSW1xx switch family

net: dsa: add driver for MaxLinear GSW1xx switch family

Add driver for the MaxLinear GSW1xx family of Ethernet switch ICs which
are based on the same IP as the Lantiq/Intel GSWIP found in the Lantiq VR9
and Intel GRX MIPS router SoCs. The main difference is that instead of
using memory-mapped I/O to communicate with the host CPU these ICs are
connected via MDIO (or SPI, which isn't supported by this driver).
Implement the regmap API to access the switch registers over MDIO to allow
reusing lantiq_gswip_common for all core functionality.

The GSW1xx also comes with a SerDes port capable of 1000Base-X, SGMII and
2500Base-X, which can either be used to connect an external PHY or SFP
cage, or as the CPU port. Support for the SerDes interface is implemented
in this driver using the phylink_pcs interface.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Tested-by: Alexander Sverdlin <alexander.sverdlin@siemens.com>
Link: https://patch.msgid.link/b567ec1b4beb08fd37abf18b280c56d5d8253c26.1762170107.git.daniel@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>