ARM: dt: tegra: ventana: add regulators

Ventana uses a TPS6586x regulator. Instantiate this, and hook up a
couple of fixed GPIO-controlled regulators too.

The data was chosen to match the PMIC HW defaults, with the following
exception:

ldo6: The HW default is 2.85v. The schematics are unlabelled. Internal
research indicates that 1.8v is correct. Our downstream kernel also uses
1.8v.

Portions based on work by Laxman Dewangan <ldewangan@nvidia.com>

Signed-off-by: Stephen Warren <swarren@nvidia.com>