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arm64: dts: imx8mq-librem5: set charger boost-max-current to 1,2A

@guido.gunther initially set this to 1,5A and @eric.kuzmenko suggested to use 1,2A. Here's his email:

Our limiting factor here would be the maximum discharge current rating of the battery which is 4.5A. So the battery should be able to provide around 13.5W when it's dead (at 3.0V). If the boost current is set to 1.5A then that means we would be able to source 5V*1.5A = 7.5W. Which means that when the battery is dead we would have 13.5W - 7.5W = 6W.

This 6W figure is barely enough to run the phone, sometimes it requires more than 6W to operate.

While the battery's max discharge current rating is 4.5A, its discharge overcurrent protection wouldn't kick in until 6.0A, so using the battery between 4.5A and 6.0A could deteriorate the life of the battery.

So do we really need be able to provide 7.5W? This would be enough to power another Librem 5 and then some.

I suggest it is reduced to 1.2A (6W), unless a case can be made that it's just not enough.

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