drivers: adc: stm32: simplify adc dma enable function

Remove specific cases for H7 and U5: group them together and only call a
single function. ADC3 of H72x/H73x and ADC4 of U5 are different from other
ADC of their series, and have dedicated functions in the LL for enabling
DMA, but they're doing the exact same operation as
LL_ADC_REG_SetDataTransferMode.

Incidentally, this change allows H7A/H7B to use the DMA (it seems to have
been missed before).

Last, this change enables the DMA support for F1x ADC.

Signed-off-by: Guillaume Gautier <guillaume.gautier-ext@st.com>