Bluetooth: btmrvl: add DT bindings documentation

Calibration data can be downloaded through device tree method. This
patch adds the documentation. Also, instead of searching device tree
node by name using of_find_node_by_name() API, let's use
for_each_compatible_node().

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Cathy Luo <cluo@marvell.com>
Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>