ARM: hisilicon: hikey: Add hikey & hi6220 dts from v4.6-rc3.

Import the upstream kernel dts into U-Boot. Currently
only serial is supported, but a lot more DT changes are
queued for v4.7.

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Reviewed-by: Tom Rini <trini@konsulko.com>