am3517evm: change console device from ttyS2 to ttyO2

the serial device names have been changed from ttySx to ttyOx, so the
console device name should be also changed to support the latest kernel
versions.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Acked-by: Vaibhav <Hiremath&lt;hvaibhav@ti.com>