fmcadc3: Add a chip-select decoder to support up to 7 devices

This is required for the fmcadc3 board as it features 7 chips all
connected to the same SPI bus (ad9528, ad9234 x2, ada4961 x4).

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
fmcadc3: Fix pre-processing of ADC data

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>