dt-bindings: iio: adc: Add binding for ADC on pms405 PMIC

PMS405 contains a variant of the spmi-adc-rev2 ADC. Create a new
compatible inorder to handle any differences.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Amit Kucheria <amit.kucheria@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>