mlxsw: minimal: Add I2C support for Mellanox ASICs

Add I2C access support for Mellanox ASICs:
- Virtual Protocol Interconnect switches SwitchX, SwitchX2,
  providing InfiniBand, Ethernet and Fibre Channel connectivity;
- Infiniband switches SwitchIB, SwitchIB2:
- Ethernet switch Spectrum.

Example of probing activation:
echo mlxsw_minimal 0x48 > /sys/bus/i2c/devices/i2c-2/new_device

Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>