omap: mmc: split out init for 2420

The MMC controller on the OMAP2420 is different from those
on the OMAP2430, OMAP3 and OMAP4 families - all of the latter
are identical. The one on the OMAP2420 is closer to that
on OMAP1 chips.

Currently, the n8x0 is the only OMAP2420 platform supported
in mainline which registers the MMC controller. Upcoming
changes to register the controllers using hwmod data are
potentially invasive. To reduce the risk, separate out the
2420 controller registration from the common init function
and update its only user. Also seperating out mux settings
for OMAP2420.

Signed-off-by: Anand Gadiyar <gadiyar@ti.com>
Signed-off-by: Kishore Kadiyala <kishore.kadiyala@ti.com>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Madhusudhan Chikkature <madhu.cr@ti.com>
Cc: Chris Ball <cjb@laptop.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>