ARM: dts: AM33XX: add ethernet alias's for am33xx

Set the alias for ethernet0 and ethernet1 so that uBoot
can set the MAC address appropriately.

Currently u-boot cannot find the alias and there for does
not set the MAC address.

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Tested-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: Benoit Cousson <bcousson@baylibre.com>