regulator: Add MPS MPQ4210 buck-boost regulator driver

The MPQ4210 is a 40V synchronous four-switch buck-boost controller with
an I2C interface. Add a driver exposing voltage control and enable
control through the regulator interface.

The output voltage is programmed by an 11-bit feedback reference DAC with
a 1mV step, split across REF_LSB[2:0] and REF_MSB[7:0], and is then
scaled by the external feedback divider described in the device tree.
Only 0.3V to 2.047V of that range is specified, so linear_min_sel holds
the driver to it and the lower selectors are not offered. Scaled by the
divider, that floor is not zero: 4.2V on a board with a gain of 14, and
the core narrows regulator-min-microvolt to it.

The same divider ratio applies to the slew rate, so the four reference
slew rates that the Control 1 SR field selects are scaled into a
per-device ramp_delay_table and the field is exposed through
regulator_set_ramp_delay_regmap(). ramp_delay is initialised from the
rate SR is currently programmed for, so the core waits in proportion to
the size of each change; a board that sets regulator-ramp-delay
reprograms SR and the core uses that value instead.

Enabling follows the start-up sequence in the datasheet: commit the
reference with the GO bit, wait 200ms, then set ENPWR. Control 1 bit 2 is
documented as reserved but has to be set before the controller starts up,
so probe sets it. The controller does not respond on the bus while EN is
deasserted, so the enable GPIO is claimed before the first register
access.

Link: https://www.monolithicpower.com/en/mpq4210.html
Signed-off-by: Tapio Reijonen <tapio.reijonen@vaisala.com>
Link: https://patch.msgid.link/20260910-mpq4210-regulator-v1-2-d37e208dfc8d@vaisala.com
Signed-off-by: Mark Brown <broonie@kernel.org>

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