ARM: OMAP2+: Add DT init code for DPI displays and make omap3 LDP to use it

In order to make the remaining omap3 boards usable with device
tree, we need to initialize the DPI based displays using pdata
until we have the DSS bindings in place.

At least board-am3517evm.c, board-cm-t35.c, board-devkit8000.c,
board-ldp.c and board-overo.c all use fairly similar DPI setup,
so let's add generic code for it. And let's make omap3 LDP to
use it and also initialize the ads7846 until we have a binding
for it.

Note that at least LDP needs the TWL bug fix posted as
"gpio: twl4030: Fix regression for twl gpio output" for the
LCD backlight to power.

Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>