documentation: docbook: document process of writing an musb glue layer

Document the process of writing an musb glue layer by taking the
Ingenic JZ4740 glue layer as an example, as it seems more simple than
most glue layers due to the basic feature set of the JZ4740 USB device
controller.

Signed-off-by: Apelete Seketeli <apelete@seketeli.net>
Signed-off-by: Felipe Balbi <balbi@ti.com>