dt-bindings: iio: adc: adi,ad7606: fix dt_schema validation warning

dt-bindings: iio: adc: adi,ad7606: fix dt_schema validation warning

Fix following dt_schema warning when offload is used:

  DTC [C] arch/arm/boot/dts/xilinx/zynq-zed-adv7511-ad7606.dtb
/home/angelo/dev-baylibre/linux-iio/arch/arm/boot/dts/xilinx/zynq-zed-adv7511-ad7606.dtb: adc@0: 'oneOf' conditional failed, one must be fixed:
	'interrupts' is a required property
	'io-backends' is a required property
	from schema $id: http://devicetree.org/schemas/iio/adc/adi,ad7606.yaml#

There isn't any reason that we couldn't have interrupts wired up at the
same time we are using io-backends or SPI offload, so dropping off the
related "oneOf" block entirely.

Fixes: 81fe5529e812 ("dt-bindings: iio: adc: adi,ad7606: add SPI offload properties")
Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/20250526-wip-bl-ad7606-dtschema-fixes-v2-1-9bd56d039489@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>