drivers: adc: STM32: add support for oversampling

The STM32 G0, G4, H7, L0, L4, WB and WL series have hardware support for
oversampling. This patch adds support for it, using the oversampling
value provided in the adc sequence. The result is shifted right
accordingly to not change the resolution of the measured value, like it
is done on other ADC drivers.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>