mmc: omap: Remove cpu_is_omap usage from the driver

This is needed for the ARM common zImage support.

We can use the existing slot features to pass omap1
specific options to the driver. For omap2 we don't
want to pass anything new as that will be eventually
moved to use device tree based init.

Note that this patch depends on earlier patch that
moves plat/mmc.h into include/linux/platform_data.

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