net: davinci_emac: drop support for unused PHYs

The boards with SoCs from the DaVinci DM* family used to come with
different PHYs that needed special support implemented in mach-davinci.

Since the support for these chips has long been removed, we can now
drop this unnused code from the emac driver.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>