MAINTAINERS: add include/dt-bindings/usb to USB SUBSYSTEM

Maintainers of the directory Documentation/devicetree/bindings/usb
are also the maintainers of the corresponding directory
include/dt-bindings/usb.

Add the file entry for include/dt-bindings/usb to the appropriate
section in MAINTAINERS.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20220613124647.32019-1-lukas.bulwahn@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>