power: bq24735-charger: document 'ti,external-control' option

Implement an 'external-control' option for when the charger
shouldn't be configured by the kernel.

Signed-off-by: Jonathan Tinkham <sctincman@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sre@kernel.org>