ARM: tegra: Add common device-tree base for Tegra30 ASUS Transformers

Add common DTSI for Tegra30 ASUS Transformers. It will be used by multiple
device-trees of ASUS devices. The common part initially was born out of
the ASUS TF300T tablet's device-tree that was created by Michał Mirosław.
It was heavily reworked and improved by Svyatoslav Ryhel, Maxim Schwalm,
Ion Agorria et al.

[digetx@gmail.com: factored out common part into separate patch and wrote commit message]
Co-developed-by: Ion Agorria <ion@agorria.com>
Signed-off-by: Ion Agorria <ion@agorria.com>
Co-developed-by: Maxim Schwalm <maxim.schwalm@gmail.com>
Signed-off-by: Maxim Schwalm <maxim.schwalm@gmail.com>
Co-developed-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
[treding@nvidia.com: cosmetic fixups]
Signed-off-by: Thierry Reding <treding@nvidia.com>