board: asus: transformer: add ASUS Transformer T30 family support

The ASUS Transformer T30 family are 2-in-1 detachable tablets
and AiO developed by ASUS that run the Android operating system
(TF600T runs Windows RT and P1801-T runs Android and Windows).
The T30 Transformers feature a 10.1-inch display (apart P1801-T),
an Nvidia Tegra 3 quad-core chip, 1/2 GB of RAM, and 16/32 GB of
storage. Transformers board derives from Nvidia Cardhu development
board.

This patch brings support for 7 known Transformer devices:
- ASUS Transformer Prime TF201
- ASUS Transformer Pad TF300T/TF300TG/TF300TL
- ASUS VivoTab RT TF600T (Windows RT based)
- ASUS Transformer Infinity TF700T
- ASUS Portable AiO P1801-T

Tested-by: Andreas Westman Dorcsak <hedmoo@yahoo.com> # all devices
Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>