adc: add Amlogic Meson SAR ADC driver

This patch adds the driver for the Amlogic Meson Successive Approximation
Register (SAR) A/D Converter based on the Linux IIO driver thanks to the
great work of Martin Blumenstingl.
The driver has been adapted to U-Boot and the ADC UClass.

This patch depends on the regmap "regmap: add regmap_update_bits() helper"
patch and has been tested using the newly introducted "adc" CLI command
in the "cmd: add ADC cli commands" patch.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>