net: dsa: ocelot: build felix.c into a dedicated kernel module

The build system currently complains:

scripts/Makefile.build:252: drivers/net/dsa/ocelot/Makefile:
felix.o is added to multiple modules: mscc_felix mscc_seville

Since felix.c holds the DSA glue layer, create a mscc_felix_dsa_lib.ko.
This is similar to how mscc_ocelot_switch_lib.ko holds a library for
configuring the hardware.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: Colin Foster <colin.foster@in-advantage.com>
Link: https://lore.kernel.org/r/20230125145716.271355-1-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>