power: supply: axp20x: add missing include bitops.h

The axp20x_usb_power driver uses BIT() operations but lacks the include
for it. Include the bitops.h header file.

Signed-off-by: Olliver Schinagl <oliver@schinagl.nl>
Signed-off-by: Priit Laes <plaes@plaes.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>