video: tegra20: dsi: add ganged mode support

Implement ganged mode support for the Tegra DSI driver. The DSI host
controller to gang up with is specified via a phandle in the device tree
and the resolved DSI host controller used for the programming of the
ganged-mode registers.

Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>