dt-bindings: power: supply: bq25890: Indent example with tabs

Fix example indentation to tabs to follow generic Linux coding style.
This avoids copying the space indentation to DTS when re-using the
example.
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>