arm64: dts: hi3660: add spi device nodes

Add spi2 and spi3 device nodes for hi3660, and enable them for hikey960.

On HiKey960:
 - SPI2 is wired out through low speed expansion connector.
 - SPI3 is wired out through high speed expansion connector.

Signed-off-by: Wang Xiaoyin <hw.wangxiaoyin@hisilicon.com>
Signed-off-by: Guodong Xu <guodong.xu@linaro.org>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>