phy: kona2: use 'select GENERIC_PHY' in Kconfig

All other phy drivers use 'select', while this one is
the only one to use 'depends on'. This is not a bug, just
slightly inconsistent, so let's change it to do things the
same way as everyone else.

We may also want to turn GENERIC_PHY into a silent option
that only ever gets turned on if another driver needs it.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Kishon Vijay Abraham I <kishon@ti.com>
Cc: Matt Porter <mporter@linaro.org>
Cc: Felipe Balbi <balbi@ti.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>