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dts: arm64: imx8mq-librem5: Set the DVS voltages lower

Sebastian Krzyszkowiak requested to merge sebastian.krzyszkowiak/linux-next:undervoltage into pureos/byzantium

They're still in the operating range according to i.MX 8M Quad datasheet. There's some headroom added over minimal values to account for voltage drop.

Operational ranges (min - typ - max [selected]):

 - VDD_SOC (BUCK1): 0.81 - 0.9 - 0.99 [0.88]
 - VDD_ARM (BUCK2): 0.81 - 0.9 - 1.05 [0.84] (1000MHz)
                    0.90 - 1.0 - 1.05 [0.93] (1500MHz)
 - VDD_GPU (BUCK3): 0.81 - 0.9 - 1.05 [0.85] (800MHz)
                    0.90 - 1.0 - 1.05 [ -- ] (1000MHz)
 - VDD_VPU (BUCK4): 0.81 - 0.9 - 1.05 [ -- ] (550/500/588MHz)
                    0.90 - 1.0 - 1.05 [0.93] (660/600/800MHz)

Idle power consumption doesn't appear to be influenced much, but a simple load test (cat /dev/urandom | pigz - > /dev/null combined with running Animatch) seems to show about 0.3W of difference.

Care is advised, as there may be differences between each units in how low can they be undervolted - in my experience, reaching that point usually makes the phone fail to boot. In my case, it appears that my Birch phone can go down the most.

This is a somewhat conservative set of values that I've seen working well on all my devices; I haven't tried very hard to optimize it, so more experiments are welcome.

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