ARM: tegra: nyan: Use external control for bq24735 charger

Nyan boards come with an embedded controller that controls when to
enable and disable the charge. Thus, it should not be left up to the
kernel to handle that.

Using the ti,external-control property allows specifying this use-case.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Signed-off-by: Thierry Reding <treding@nvidia.com>