boards: tdk_robokit1: chip select fixes

PC31 is used by the onboard adc connected to SPI as the chip select line
and needs to be unused by afec. Disable afec1.

The adc was using reg1 which implies using the second hardware chip select
which is also not quite right. For software chip select use reg 0 instead.

Signed-off-by: Tom Burdick <thomas.burdick@intel.com>