OMAPDSS: create DPI & SDI devices

We currently have separate device/driver for each DSS HW module. The DPI
and SDI outputs are more or less parts of the DSS or DISPC hardware
modules, but in SW it makes sense to represent them as device/driver
pairs similarly to all the other outputs. This also makes sense for
device tree, as each node under dss will be a platform device, and
handling DPI & SDI somehow differently than the rest would just make the
code more complex.

This patch modifies arch/arm/mach-omap2/display.c to create platform
devices for DPI and SDI, and later patches will implement driver for
them.

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