iio: adc: stm32: fix maximum clock rate for stm32mp15x

Change maximum STM32 ADC input clock rate to 36MHz, as specified
in STM32MP15x datasheets.

Fixes: d58c67d1

 ("iio: adc: stm32-adc: add support for STM32MP1")
Signed-off-by: Olivier Moysan <olivier.moysan@foss.st.com>
Reviewed-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Link: https://lore.kernel.org/r/20220609095234.375925-1-olivier.moysan@foss.st.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>