cs89x0: rework driver configuration

There are two drivers in the cs89x0 file, with the CONFIG_CS89x0_PLATFORM
symbol deciding which one is getting built. This is somewhat confusing
and makes it more likely ton configure a driver that works nowhere.

Split up the Kconfig option into separate ISA and PLATFORM drivers,
with the ISA symbol explicitly connecting to the static probing in
drivers/net/Space.c

The two drivers are still mutually incompatible at compile time,
which could be lifted by splitting them into multiple files,
but in practice this will make no difference.

The platform driver can now be enabled for compile-testing on
non-ARM machines.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>