ARM: dts: imx6dl-yapp4: Add ethernet aliases

Add aliases for ethernet switch ports to allow bootloader to fix
MAC addresses to the ones stored in onboard configuration EEPROM.

Ursa has only one ethernet port populated (eth2) so alias for
the first port has to be removed on this board.

Signed-off-by: Michal Vokáč <michal.vokac@ysoft.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>