regulator: da9121: Allow caching BUCK registers

Some BUCK registers may change without software writes when GPIO pins
are configured for functions DVC/RELOAD/EN. If the board does
not use these pin-controlled features, caching is possible.

Caching BUCK registers removes unnecessary I2C reads when performing
register updates. For example, updating regulator mode can result in
two I2C reads, one from the regulator core regulator_set_mode() and one
from the DA9121 driver, where da9121_buck_set_mode() uses
regmap_update_bits() (read/modify/write).

Check for the optional DT property dlg,no-gpio-control. When present,
select the regmap configuration that does not mark the BUCK1 register
block (DA9121_REG_BUCK_BUCK1_0..DA9121_REG_BUCK_BUCK1_6) as volatile, so
that regmap can cache BUCK1 registers and avoid unnecessary I2C reads.
The property dlg,no-gpio-control is required to ensure that BUCK1
registers can be cached, as the absence of relevant GPIO DT properties
does not imply that the RELOAD/DVC/EN GPIO functions are unused. These
functions are provided by DA91xx GPIO pins and may be controlled by
external hardware without corresponding GPIO DT properties. The
dlg,no-gpio-control property explicitly indicates that none of these
GPIO functions are used.

The dlg,no-gpio-control property is mutually exclusive with
enable-gpios, regardless of whether the referenced GPIO is connected to
a GPIO pin or the IC_EN pin, since pulling IC_EN low powers down the
regulator and registers are reinitialized at startup, leaving cached
values stale.

Co-developed-by: Waqar Hameed <waqar.hameed@axis.com>
Signed-off-by: Waqar Hameed <waqar.hameed@axis.com>
Signed-off-by: André Svensson <andre.svensson@axis.com>
Link: https://patch.msgid.link/20260320-no-gpio-control-v2-2-dbc938e462cb@axis.com
Signed-off-by: Mark Brown <broonie@kernel.org>

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