ARM: dts: tegra: add Tegra20 NAND flash controller node

Add basic controller device tree node to be extended by
individual boards. Use the assigned-clocks mechanism to set
NDFLASH clock to a sensible default rate of 150MHz.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>