dt/bindings: Add binding for the DA8xx MUSB driver

DT binding for the TI DA8xx/OMAP-L1x/AM17xx/AM18xx MUSB driver.

Signed-off-by: Petr Kulhavy <petr@barix.com>
Signed-off-by: Alexandre Bailon <abailon@baylibre.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>