Platform/RPi3: Provide "ethernet[0]" aliases in device tree

Older device trees tend to use the alias "ethernet".  Newer device
trees tend to use "ethernet0" since older versions of U-Boot would
skip aliases that do not include an index number.  See, for example,
Linux kernel commit 10b6c0c ("ARM: dts: bcm2835: add index to the
ethernet alias") and U-Boot commit f8e57c6 ("fdt_support: Fixup
'ethernet' aliases not ending in digits").

Ensure that both "ethernet" and "ethernet0" aliases are present within
the device tree, to provide compatibility with operating systems that
expect either alias, and with device trees that contain either (or
both) aliases.

Signed-off-by: Michael Brown <mbrown@fensystems.co.uk>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>