OMAPDSS: DT: Get source endpoint by matching reg-id

In omapdss_of_find_source_for_first_ep, we retrieve a source endpoint's DT node,
and then see what omapdss output has the matching device_node pointer in
omap_dss_find_output_by_node.

For all DPI and SDI outputs, the device_node pointer is set as the parent's DSS
device_node pointer. If the source is one of these outputs, the above method
won't work.

To get the correct output for ports within DSS(and in other cases in the future,
where multiple ports might be under one device), we require additional
information which is exclusive to the output port.

We create a new field in omap_dss_device called 'port_num', this provides port
number of the output port corresponding to this device. When searching for the
source endpoint in DT, we extract the 'reg' property from the port corresponding
to the endpoint source. From the list of registered outputs, we pick out that
output which has both dev->of_node and port_num matching with the device_node
pointer and 'reg' of the source endpoint node from DT.

For encoder blocks(the ones which have both an input and output port), we need
to set the port_num as the 'reg' property for the output port as defined in the
DT bindings. We set port_num to 1 in the tfp410 and tpd12s015 encoder drivers.

Signed-off-by: Archit Taneja <archit@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>