arm64: dts: hikey960: Add pstore support

This patch reserves some memory in the DTS and sets up a
pstore device tree node to enable pstore support on HiKey960.

Cc: John Stultz <john.stultz@linaro.org>
Signed-off-by: Guodong Xu <guodong.xu@linaro.org>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>