ARM: dts: zx: add an initial zx296702 dts and doc

Add initial dts file and document for ZX296702 and board ZX296702-AD1.
More peripherals will be added later.

Signed-off-by: Jun Nie <jun.nie@linaro.org>
Signed-off-by: Kevin Hilman <khilman@linaro.org>
diff --git a/arch/arm/boot/dts/Makefile b/arch/arm/boot/dts/Makefile
index 86217db..4814c6b 100644
--- a/arch/arm/boot/dts/Makefile
+++ b/arch/arm/boot/dts/Makefile
@@ -660,6 +660,7 @@
 	mt6592-evb.dtb \
 	mt8127-moose.dtb \
 	mt8135-evbp1.dtb
+dtb-$(CONFIG_ARCH_ZX) += zx296702-ad1.dtb
 endif
 
 always		:= $(dtb-y)