The AXP209 compatible was used in Device Trees and the driver, but it
was never documented.

Cc: Chen-Yu Tsai <wens@csie.org>
Cc: linux-pm@vger.kernel.org
Cc: Sebastian Reichel <sre@kernel.org>
Signed-off-by: NMaxime Ripard <maxime@cerno.tech>
Reviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NSebastian Reichel <sebastian.reichel@collabora.com>