mmc: omap: Fix I2C dependency and make driver usable with device tree

Some features can be configured by the companion I2C chips,
which may not be available at the probe time. Fix the issue
by returning -EPROBE_DEFER when the MMC controller slots
are not configured.

While at it, let's also add minimal device tree support so
omap24xx platforms can use this driver without legacy mode
since we claim to support device tree for mach-omap2 based
systems.

Although adding the minimal device tree support is not strictly
a fix, it does remove one of the last blockers for dropping a
bunch of legacy platform data for mach-omap2.

Cc: linux-mmc@vger.kernel.org
Acked-by: Chris Ball <cjb@laptop.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>