davinci_emac: move arch-independent defines to separate header

DaVinci EMAC is found not only on DaVinci SoCs but on some OMAP3 SoCs
also. This patch moves common defines from arch-davinci/emac_defs.h to
drivers/net/davinci_emac.h

DaVinci specific PHY drivers hacked to include the new header. We might
want to switch to phylib in future.

Signed-off-by: Ilya Yanok <yanok@emcraft.com>