arm64: dts: hi3660: Add uarts nodes

Add nodes uart0 to uart4 and uart6 for hi3660 SoC.
Enable uart3 and uart6, disable uart5, in hikey960 board dts.

On HiKey960:
 - UART6 is used as default console, and is wired out through low speed
         expansion connector.
 - UART3 has RTS/CTS hardware handshake, and is wired out through low
         speed expansion connector.
 - UART5 is not used in commercial launched boards. So disable it.
 - UART4 is connected to Bluetooth, WL1837.

Signed-off-by: Chen Feng <puck.chen@hisilicon.com>
Signed-off-by: Wang Xiaoyin <hw.wangxiaoyin@hisilicon.com>
Signed-off-by: Guodong Xu <guodong.xu@linaro.org>
Reviewed-by: Zhangfei Gao <zhangfei.gao@hisilicon.com>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>