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>
7 files changed