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

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.