documentation: Add nvmem bindings documentation

This patch add the bindings document of Amlogic eFuse driver.

Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Carlo Caione <carlo@endlessm.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>