arm/dt: Tegra: Add pinmux node to tegra20.dtsi

Add a pinmux node to tegra20.dtsi in order to instantiate the future
pinmux device.

v2: Specify reg property precisely; don't just point at the whole APB_MISC
register range.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Olof Johansson <olof@lixom.net>