dt-bindings: thermal: Add DT bindings documentation for Amlogic Thermal

Adding the devicetree binding documentation for the Amlogic temperature
sensor found in the Amlogic Meson G12A and G12B SoCs.

Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org>
Tested-by: Christian Hewitt <christianshewitt@gmail.com>
Tested-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Guillaume La Roque <glaroque@baylibre.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20191004090114.30694-2-glaroque@baylibre.com