net: Kconfig:make PHY_GIGE and individual Micrel PHYs selectable

net: Kconfig:make PHY_GIGE and individual Micrel PHYs selectable

This change migrate the following configuration options for Kconfig:
 * PHY_GIGE, indicates that a controller (with an appropriate PHY) is
   Gigabit capable and enables extra support in the miiutil for
   parsing the status of Gigabit PHYs
 * adds configuration options for Micrel KSZ9021 and KSZ9031 GbE PHYs,
   which previously had to enabled through a board-specific config file

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>