arm64: dts: hisilicon: hikey970-pmic: fix regulator cells properties

The Hi6421 PMIC regulator child nodes do not have unit addresses so drop
the incorrect '#address-cells' and '#size-cells' properties.

Fixes: 6219b20e ("arm64: dts: hisilicon: Add support for Hikey 970 PMIC")
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>