arm64: dts: rockchip: Rename vcc12v_sys into dc_12v for roc-rk3399-pc

arm64: dts: rockchip: Rename vcc12v_sys into dc_12v for roc-rk3399-pc

It is always better practice to follow regulator naming conventions
as per the schematics for future references.

This would indeed helpful to review and check the naming convention
directly on schematics, both for the code reviewers and the developers.

So, rename vcc12v_sys into dc_12v as per rk3399 power tree as per
roc-rk3399-pc schematics.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Link: https://lore.kernel.org/r/20190919052822.10403-6-jagan@amarulasolutions.com
Signed-off-by: Heiko Stuebner <heiko@sntech.de>