arm64: dts: rockchip: Rename vcc_sys into vcc5v0_sys on rk3399-rock960

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

So, rename vcc_sys into vcc5v0_sys as per rk3399 power diagram of
rock960 schematics.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Akash Gajjar <akash@openedev.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>