Documentation: dt: Add binding info for jz4740-musb driver

This commit adds documentation for the device-tree bindings of the
jz4740-musb driver, which provides support for the USB gadget mode
of the JZ4740 and similar SoCs from Ingenic.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>