ARM: OMAP2: MMC: include mmc-omap platform header directly

Only a few files really need that platform header. When later splitting
omap_mmc_platform_data into omap_mmc and omap_mmc_hs, those files
declaring an hs mmc platform data will have to change the platform
include, which is a good sanity check.
Also removing omap242x_init_mmc, which is not used anywhere, checked
with grep.

Signed-off-by: Andreas Fenkart <afenkart@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>