dt-bindings: net: qrtr: Add MHI Device transport bindings

The QRTR MHI DEV driver acts as a transport driver for a device that is
configured as a pcie endpoint. Document the bindings for this qrtr
transport.

Change-Id: I289b858b094d7e3e62a5ded61d42cde04518093c
Signed-off-by: Chris Lew <clew@codeaurora.org>