dt-bindings: iio: dac: adi-axi-adc: fix ad7606 pwm-names

dt-bindings: iio: dac: adi-axi-adc: fix ad7606 pwm-names

Fix make dt_binding_check warning:

DTC [C] Documentation/devicetree/bindings/iio/adc/adi,axi-adc.example.dtb
.../adc/adi,axi-adc.example.dtb: adc@0: pwm-names: ['convst1'] is too short
    from schema $id: http://devicetree.org/schemas/iio/adc/adi,ad7606.yaml#

Add "minItems" to pwm-names, it allows to use one single pwm when
connected to both adc conversion inputs.

Fixes: 7c2357b10490 ("dt-bindings: iio: adc: ad7606: Add iio backend bindings")
Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20250129-wip-bl-ad7606_add_backend_sw_mode-v3-1-c3aec77c0ab7@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>