drivers: adc: lmp90xxx: make adc_context_start_sampling() ISR safe

Introduce a dedicated function for performing an entire LMP90xxx ADC
channel read and sample all channels in one go in the ADC acquisition
thread.

This removes the SPI transactions from adc_context_start_sampling()
which can be called in k_timer ISR context for consecutive ADC reads.

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>