arm64: dts: librem5-devkit: add the regulators for DVFS

Specify which regulator is used for cpufreq DVFS.

Signed-off-by: Angus Ainslie (Purism) <angus@akkea.ca>
Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>