arm64: dts: librem5-devkit: set mipi power supply regulator to always on

This fixes one particular issue that blocks the devkit from properly
doing S3 suspend and resume. For some background, see
https://source.puri.sm/Librem5/linux-next/issues/83
1 job for for-angus-v5.3/ldo5 in 29 minutes and 4 seconds (queued for 3 seconds)
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