HWMON: Add JZ4740 ADC driver

Add support for reading the ADCIN pin of the ADC unit on JZ4740 SoCs.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: lm-sensors@lm-sensors.org
Acked-by: Jean Delvare <khali@linux-fr.org>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/1425/
Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>