net: phy: micrel: Separate KSZ9000 drivers from KSZ8000 drivers

The KS8721BL and KSZ9021 PHYs are software-incompatible, yet they
share the same ID. Drivers for bothe PHYs cannot safely coexist, so
the solution was to use #ifdefs to select between the two drivers.

As a result KSZ9031, which has a unique ID, is now caught in the
crossfire. Unless CONFIG_PHY_MICREL_KSZ9031 is defined, the KSZ9031
will not function properly, as some essential configuration code is
ifdef'd-out.

To prevent such situations, move the KSZ9000 drivers to a separate
file, and place them under a separate Kconfig option. While it is
possible to enable both KSZ8000 and KSZ9000 drivers at the same time,
the assumption is that it is highly unlikely for a system to contain
both a KSZ8000 and a KSZ9000 PHY, and that only one of the drivers
will be enabled at any given time.

Signed-off-by: Alexandru Gagniuc <alex.g@adaptrum.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>