iio: adc: stm32-adc: add support of internal channels

Add support of ADC2 internal channels VDDCORE, VREFINT and VBAT.
The reserved label name "vddcore", "vrefint" and "vbat" must
be used in Device Tree channel node, to enable the corresponding
internal channel.

Note: This patch does not provide support of internal channels
for F4 and H7.

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/20211014131228.4692-6-olivier.moysan@foss.st.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>