sunxi: add gmac Ethernet support

Add support for the GMAC Ethernet controller on Allwinner A20 (sun7i)
processors. Enable for the Cubietruck.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Jens Kuske <jenskuske@gmail.com>
Signed-off-by: Ian Campbell <ijc@hellion.org.uk>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Tom Rini <trini@ti.com>
diff --git a/board/sunxi/Makefile b/board/sunxi/Makefile
index 18b1e11..cbf8f08 100644
--- a/board/sunxi/Makefile
+++ b/board/sunxi/Makefile
@@ -9,4 +9,5 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 obj-y	+= board.o
+obj-$(CONFIG_SUNXI_GMAC)	+= gmac.o
 obj-$(CONFIG_CUBIETRUCK)	+= dram_cubietruck.o