regulator: TPS6287X: Use min/max uV to get VRANGE

Changing voltage might ignore slew rate and cause a current surge.

With current implementation the driver will get the regulator to change
the voltage range used during run time. According to communication I
have had with Texas Instruments, this is not intended, since the
Dynamic Voltage Scaling in the hardware is only designed to work
within a voltage range. The current implementation will therefore
ignore the slew rate that is defined in devicetree when the voltage
range is changed during use.

The current implementation will always select a voltage in the most
accurate range that can reach that voltage even though multiple ranges
are able to reach that voltage. There are 4 Voltage ranges with the
following reach:
0b00: 0.4-0.71875V      (1.25mV step size)
0b01: 0.4-1.0375V       (2.5mV)
0b10: 0.4-1.675V        (5mV)
0b11: 0.8-3.3V          (10mV)
This in practice means that a change from below to above 0.71875V will
use the smallest range(0b00) for the values below and the second
smallest range(0b01) for the voltages above (Up to 1.675V). I have
timed how long it takes to go from below 0.71875V to above. The
increase was 100mV which, with the slew rate set to 1250µV/µs. This
in theory should take 80µs to do. With the current implementation, it
takes 10µs on my hardware. Doing the same test with the slew rate set
to 5000µV/µs, which should take 20µs, also only takes 10µs to do on
my hardware. Not only is this not in line with the technical
specification for the regulator. It also causes a current surge. Which
when calculating the output current, as described in the technical
specification, compared to what I could observe on my hardware the real
output is ~1A higher (~1.2A) than what I calculated it to be(~0.2A).

I tested also transitioning from a bigger to a smaller range, and the
results were the same.

Instead, let's limit the voltage range to a single one, which is in
line with the intended use of the regulator. This is done by looking
up the minimum and maximum requested voltage specified in devicetree.

Signed-off-by: Jonas Andreasson <jonas.andreasson@axis.com>
Link: https://patch.msgid.link/20250121-tps-fix-v2-1-50cc4d0f1635@axis.com
Signed-off-by: Mark Brown <broonie@kernel.org>

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