ARM: tegra: Fix typoed ams,ext-control properties

The property for enabling external rail control on the AS3722 is
ams,ext-control, not ams,external-control. Since the external rail
control property was previously being ignored, LP1 suspend on these
boards wasn't actually turning the CPU rail off at all.

Signed-off-by: Tuomas Tynkkynen <ttynkkynen@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>