arm64: dts: renesas: salvator-common: Add cpu-supply property to a57_0 node

Add the cpu-supply property to the a57_0 node, so Dynamic Voltage and
Frequency Scaling (DVFS) can change the CPU core voltage.

Signed-off-by: Dien Pham <dien.pham.ry@renesas.com>
Signed-off-by: Takeshi Kihara <takeshi.kihara.df@renesas.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20210326105009.1574424-2-geert+renesas@glider.be