dt-bindings: power: supply: bq25890: Document required interrupt

The driver requires interrupts (fails probe if it is not provided) so
document this requirement in bindings.

Fixes: 4aeae9cb ("power_supply: Add support for TI BQ25890 charger chip")
Cc: <stable@vger.kernel.org>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>