drm/mipi_dsi: Get DSI host by DT device node

mipi_dsi_devices are inherently aware of their host because they
share a parent-child hierarchy in the device tree.

Non-dsi drivers that create a dummy dsi device don't have this data.
In order to get this information, they require to a phandle to the dsi
host in the device tree.

Maintain a list of all the hosts DSI that are currently registered.

This list will be used to find the mipi_dsi_host corresponding to the
device_node passed in of_find_mipi_dsi_host_by_node.

Signed-off-by: Archit Taneja <architt@codeaurora.org>