dt-bindings: reset: Add bindings for the Meson-AXG SoC Reset Controller

Add DT bindings for the Meson-AXG SoC Reset Controller include file,
and also slightly update documentation.

Signed-off-by: Yixun Lan <yixun.lan@amlogic.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>