board: asus: grouper: add Google Nexus 7 (2012) support

Nexus 7 is a mini tablet computer co-developed by Google and Asus
that runs the Android operating system. The Nexus 7 features a 7"
display, an Nvidia Tegra 3 quad-core chip, 1 GB of RAM and 8/16 GB
of internal storage.

This patch brings support for all 3 known ASUS/Google devices:
- Nexus 7 (2012) E1565
- Nexus 7 (2012) PM269
- Nexus 7 (2012) 3G - tilapia

Tested-by: Andreas Westman Dorcsak <hedmoo@yahoo.com> # ASUS Grouper E1565
Tested-by: Svyatoslav Ryhel <clamor95@gmail.com> # ASUS Grouper E1565
Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>