[dif/adc_ctrl] Add header with proposed API
[dif/adc_ctrl] Add header with proposed API

This adds the DIF header for the ADC Controller IP.

Signed-off-by: Timothy Trippel <77eacc84f9735d3cbb55a271710be269fc3a5818@google.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan