iio: adc: aspeed: Add AST2700 ADC support

iio: adc: aspeed: Add AST2700 ADC support

This patch adds support for the ADCs found on the Aspeed AST2700 SoC,
which includes two instances: "ast2700-adc0" and "ast2700-adc1". While
they are functionally similar to those on AST2600, the OTP trimming data
is located at the same offset (0x820), but uses different bitfields.

Signed-off-by: Billy Tsai <billy_tsai@aspeedtech.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>