dt-bindings: net: Add schema for Qualcomm BAM-DMUX

The BAM Data Multiplexer provides access to the network data channels of
modems integrated into many older Qualcomm SoCs, e.g. Qualcomm MSM8916 or
MSM8974. It is built using a simple protocol layer on top of a DMA engine
(Qualcomm BAM) and bidirectional interrupts to coordinate power control.

The device tree node combines the incoming interrupt with the outgoing
interrupts (smem-states) as well as the two DMA channels, which allows
the BAM-DMUX driver to request all necessary resources.

Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Signed-off-by: David S. Miller <davem@davemloft.net>