iio: adc: stm32-adc: fix a wrong error message when probing interrupts

A wrong error message is printed out currently, like on STM32MP15:
- stm32-adc-core 48003000.adc: IRQ index 2 not found.

This is seen since commit 7723f4c5 ("driver core: platform: Add an
error message to platform_get_irq*()").
The STM32 ADC core driver wrongly requests up to 3 interrupt lines. It
should request only the necessary IRQs, based on the compatible:
- stm32f4/h7 ADCs share a common interrupt
- stm32mp1, has one interrupt line per ADC.
So add the number of required interrupts to the compatible data.

Fixes: d58c67d1

 ("iio: adc: stm32-adc: add support for STM32MP1")
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>