device_get_supply_regulator() only needs to be called once.
But each time there's call to adc_vxx_value() for instance, it calls
adc_vxx_platdata_update() -> device_get_supply_regulator().

So, move device_get_supply_regulator() to pre_probe() routine.

This also allows vdd_supply/vss_supply to be provided directly from
uc_pdata, e.g dt-binding variant like stm32-adc provide its own
'vref-supply'.
Signed-off-by: NFabrice Gasnier <fabrice.gasnier@st.com>
Reviewed-by: NSimon Glass <sjg@chromium.org>