net: mscc: ocelot: convert MSCC_OCELOT_SWITCH into a library

Hide the CONFIG_MSCC_OCELOT_SWITCH option from users. It is meant to be
only a hardware library which is selected by the drivers that use it
(ocelot, felix).

Since it is "selected" from Kconfig, all its dependencies are manually
transferred to the driver that selects it. This is because "select" in
Kconfig language is a bit of a mess, and doesn't handle dependencies of
selected options quite right.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>