ARM: OMAP: move generic EMAC init to separate file

AM35xx SoCs include DaVinci EMAC IP. Initialization code in
board-am3517evm.c is pretty board independent and will work for any
AM35xx based board so move this code to it's own file to be reused by
other boards.

Signed-off-by: Ilya Yanok <yanok@emcraft.com>
Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Tony Lindgren <tony@atomide.com>