arm64: dts: rockchip: rockpro64 dts make regulator more readable

rename dc12, vcc_sys, vcc1v8_pmu regulators and make it more redable as per the
schematic of rk3399-rockpro64.

Signed-off-by: Akash Gajjar <Akash_Gajjar@mentor.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>