dt-bindings: power: supply: qcom,pmi8998-charger: add bindings for smb2 driver

Add devicetree bindings for the Qualcomm PMI8998/PM660 SMB2 charger
driver.

Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>