ARM: dt: tegra: ventana: define pinmux for ddc

Tegra 2's I2C2 controller can be routed to either the PTA
or DDC pin group on Ventana. So:
- Remove the HDMI function definition of pta pingroup
- Define child i2c adapters(ddc & pta) for I2C2 controller

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