ARM: OMAP2: Use hwmod to initialize mmc for 2420

This allows us to pass dma request lines in platform data to
MMC driver the same way as we already do for omap2430 and later.

Also note that we need to only build this code if MMC_OMAP
is selected, so change Makefile accordingly and place it near
the MMC_OMAP_HS in the Makefile.

Signed-off-by: Tony Lindgren <tony@atomide.com>