arm/mxs: fix mmc device adding for mach-mx28evk

The merge commit "526b2641 Merge branch 'imx/cleanup' into imx/devel"
left a duplicated mx28_add_mxs_mmc() call, which causes the problem
below during boot.

  kobject_add_internal failed for mxs-mmc.1 with -EEXIST, don't try
  to register things with the same name in the same directory.

The patch removes this leftover and also change mmc0 adding to align
with mmc1.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>