dt-bindings: power: supply: bq25890: use one fallback compatible

BQ2589[0256] seem compatible between each other in major aspects and
remaining features are auto-detectable (by reading device revision ID
register).  Existing DTS already uses the compatibles with a fallback,
so adjust the bindings to reflect this and fix dtbs_check warning:

  arch/arm64/boot/dts/freescale/imx8mq-librem5-r2.dtb: charger@6a: compatible: ['ti,bq25895', 'ti,bq25890'] is too long

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>