tegra2: Move tegra2_mmc_init() prototype to public header.

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 <[email protected]>
Acked-by: Stephen Warren <[email protected]>
Tested-by: Stephen Warren <[email protected]>
Acked-by: Simon Glass <[email protected]>
Signed-off-by: Tom Warren <[email protected]>