net: use the same alias stem for ethernet as linux

Linux uses the prefix "ethernet" whereas u-boot uses "eth". This is from
the linux tree:

$ grep "eth[0-9].*=.*&" arch/**/*dts{,i}|wc -l
0
$ grep "ethernet[0-9].*=.*&" arch/**/*dts{,i}|wc -l
633

In u-boot device trees both prefixes are used. Until recently the only
user of the ethernet alias was the sandbox test device tree. This
changed with commit fc054d56 ("net: Introduce DSA class for Ethernet
switches"). There, the MAC addresses are inherited based on the devices
sequence IDs which is in turn given by the device tree.

Before there are more users in u-boot and both worlds will differ even
more, rename the alias prefix to "ethernet" to match the linux ones.
Also adapt the test cases and rename any old aliases in the u-boot
device trees.

Cc: David Wu <david.wu@rock-chips.com>
Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>