Documentation/fb: add documentation for sm712fb

Create the documentation for SM712. Mention all the supported modes and
how to use.
Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/fb/sm712fb.txt

+What is sm712fb?
+=================
+
+This is a graphics framebuffer driver for Silicon Motion SM712 based processors.
+
+How to use it?
+==============
+
+Switching modes is done using the video=sm712fb:... boot parameter.
+
+If you want, for example, enable a resolution of 1280x1024x24bpp you should
+pass to the kernel this command line: "video=sm712fb:0x31B".
+
+You should not compile-in vesafb.
+
+Currently supported video modes are:
+
+[Graphic modes]
+
+bpp | 640x480  800x600  1024x768  1280x1024
+----+--------------------------------------------
+  8 | 0x301    0x303    0x305    0x307
+ 16 | 0x311    0x314    0x317    0x31A
+ 24 | 0x312    0x315    0x318    0x31B
+
+Missing Features
+================
+(alias TODO list)
+
+	* 2D acceleratrion
+	* dual-head support