sunxi: Add EMAC ethernet0 alias for H3 dtsi

The sunxi ethernet address generation code looks for ethernet[0-3]
aliases to find ethernet controllers to generate MAC addresses for.

Without a valid address, the driver fails to register.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>