OMAPDSS: Add DT support to DSI

Add the code to make the DSI driver work with device tree on OMAP3 and
OMAP4.

A minor hack is needed at the moment in the DSI driver: the DSS driver
needs to know the ID number of a DSI device, as clocks are routed in
different ways to the DSI devices. At the moment we don't have any
proper way to manage this, so this patchs adds a simple lookup table
that is used to deduce the ID from the DSI device's base address.

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