add btmrvl.txt to Documentation/

This patch incorporates a lot of comments given by
Nicolas Pitre <nico@marvell.com>. Many thanks to Nicolas Pitre.
Signed-off-by: NRahul Tank <rahult@marvell.com>
Signed-off-by: NBing Zhao <bzhao@marvell.com>
Signed-off-by: NMarcel Holtmann <marcel@holtmann.org>