riscv: dts: starfive: Enable axp15060 pmic for cpufreq

The VisionFive 2 board has an embedded pmic axp15060,
which supports the cpu DVFS through the dcdc2 regulator.
This patch enables axp15060 pmic and configs the dcdc2.

Signed-off-by: Mason Huo <mason.huo@starfivetech.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>