iio: adc: axp20x_adc: add support for AXP813 ADC

The X-Powers AXP813 PMIC is really close to what is already done for
AXP20X/AXP22X.

There are two pairs of bits to set the rate (one for Voltage and Current
measurements and one for TS/GPIO0 voltage measurements) instead of one.

The register to set the ADC rates is different from the one for
AXP20X/AXP22X.

GPIO0 can be used as an ADC (measuring Volts) unlike for AXP22X.

The scales to apply to the different inputs are unlike the ones from
AXP20X and AXP22X.

Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>