arm64: dts: hisilicon: Add support for Hikey 970 PMIC

Add a device tree for the HiSilicon 6421v600 SPMI PMIC, used
on HiKey970 board.

As we now have support for it, change the fixed regulators
used by the SD I/O to use the proper LDO supplies.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>