[U-Boot] am3517_evm: Ethernet activation

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This patch continues Tom's effort in activating Ethernet on am3517_evm board
(http://git.denx.de/cgi-bin/gitweb.cgi?p=u-boot/u-boot-arm.git;a=commit;h=18a02e8050b7af165efa72325753e7880bf5567c).

I've removed #if statements, because in include/configs/am3517_evm.h EMAC is activated by default. I've also enebled
CONFIG_OMAP_GPIO (needed to turn pin 30 on).

Patch created against u-boot/u-boot-ti.git

Yegor