tegra2: Move tegra2_mmc_init() prototype to public header.

tegra2_mmc_init() is implemented by the Tegra2 MMC driver. Since most of
the Tegra2-based boards will need to call it, this commit exports it in
the new public asm/arch/mmc.h header file to prevent each board from
providing its own prototype.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Acked-by: Stephen Warren <swarren@nvidia.com>
Tested-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>