ARM: hisi: add ARCH_HISI

Since multiple ARCH configuration will be appended into mach-hisi
directory, add ARCH_HISI as common configuration for different platforms
in mach-hisi.

Signed-off-by: Haojian Zhuang <haojian.zhuang@linaro.org>
Acked-by: Wei Xu <xuwei5@hisilicon.com>
Signed-off-by: Olof Johansson <olof@lixom.net>