Rename Hisilicon HI3716 to HIX5HD2 and add dependency on ARCH_HIX5HD2.

HiX5HD2 is a SoC with dual Cortex A9 cores for STB market, and original
Hi3xxx SoC is for mobile market.

To avoid confusing the two types of SoCs and also because of different
implementation (SMP, IPs integrated and earlycon config), rename Hi3716
to HIX5HD2 and add ARCH_HIX5HD2.
Signed-off-by: Haifeng Yan <yanhaifeng@gmail.com>
Signed-off-by: Jiancheng Xue <jchxue@gmail.com>
Signed-off-by: Haojian Zhuang <haojian.zhuang@linaro.org>
Acked-by: Wei Xu <xuwei5@hisilicon.com>
[olof: fixed description typos]
Signed-off-by: Olof Johansson <olof@lixom.net>