board: asus: transformer: add ASUS Transformer T20 family support

The Asus Eee Pad Transformer family are 2-in-1 detachable/slider
tablets developed by Asus that run the Android operating system.
The Eee Pad Transformers feature a 10.1-inch (260 mm) display,
an Nvidia Tegra 2 dual-core chip, 1 GB of RAM, and 16/32 GB of storage.
Transformers board derives from Nvidia Ventana development board.

This patch brings support for all 3 known T20 Transformers:
- Asus Eee Pad Transformer TF101
- Asus Eee Pad Transformer TF101G
- Asus Eee Pad Slider SL101

Tested-by: Robert Eckelmann <longnoserob@gmail.com> # ASUS TF101
Tested-by: Antoni Aloy Torrens <aaloytorrens@gmail.com> # ASUS TF101
Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>
diff --git a/doc/board/asus/index.rst b/doc/board/asus/index.rst
index 87e535f..2b10328 100644
--- a/doc/board/asus/index.rst
+++ b/doc/board/asus/index.rst
@@ -7,4 +7,5 @@
    :maxdepth: 2
 
    grouper_common
+   transformer_t20
    transformer_t30