ads1220: Add input_mux and vref options to ADS1220 sensor (#6713)

[rogerlz]

• fix type comparison bug that stopped the sensor from initializing
• correct mismatch between docs and code for sample_rate (fixed to work same as hx71x)
• add input_mux, pga_bypass and vref options
• update configuration reference & fix typo

Klipper PR: https://github.com/Klipper3d/klipper/pull/6713