drm/msm/dsi-staging: add support for secondary display

Add secondary DSI device tree node and add it in the list of
supported connectors. On probe, create a secondary DSI device
and add it to the list of components. This is needed to drive
an independent secondary display. In case secondary display is
not supported either by device tree or boot-loader, the subsequent
component binding will be ignored and corresponding connector
and encoder will not be created. So, this approach is designed
to support both single and dual DSI displays.

CRs-Fixed: 2254681
Change-Id: Ida37a80e07cff9ecdb6613af0f755ca5a64aaad4
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>