Add support to hikey dts for the syscon-reboot-mode driver.

After trying an approach using a sram driver and node, a number
of issues cropped up which would make it so we would be
duplicating a lot of extra syscon infrastructure in order to
support mfds on sram. After talking with Bjorn, using the
syscon driver for this seems like an better choice.

Cc: Andy Yan <andy.yan@rock-chips.com>
Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
Cc: Haojian Zhuang <haojian.zhuang@linaro.org>
Cc: Guodong Xu <guodong.xu@linaro.org>
Cc: Wei Xu <xuwei5@hisilicon.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: devicetree@vger.kernel.org
Acked-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>