dt-bindings: mfd: x-powers,axp152: Document the AXP313a variant

The X-Powers AXP313a is a PMIC used on some devices with the Allwinner
H616 or H313 SoC.

According to the datasheet, the DC/DC converter PWM frequency is fixed
(to 3 MHz), so disallow the property that lets us set this frequency
for the other PMICs.

Signed-off-by: Martin Botka <martin.botka@somainline.org>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230401001850.4988-2-andre.przywara@arm.com


(cherry picked from commit 2a9e8a1a7c4caadf690e5a77fe4162c5edab4a9c)
Signed-off-by: Lee Jones <lee@kernel.org>