drivers/mmc: move spl_mmc.c to common/spl

drivers/mmc: move spl_mmc.c to common/spl

The mpc85xx repuires a special layout on the memory device that is
connected to the eSDHC controller interface. But the file spl_mmc.c
didn't handle this specfic case, there needs a special treatmen, in
the powerpc drictory. So, there is no longer to keep spl_mmc.c on
mpc85xx, CONFIG_SPL_FRAMEWORK is not set.

When CONFIG_SPL_MMC_SUPPORT is set and CONFIG_SPL_FRAMEWORK is not
set, there was an error in drivers/mmc/spl_mmc.c:

drivers/mmc/libmmc.o:(.got2+0x8): undefined reference to `spl_image'.

Now, the solution is to move the file "spl_mmc.c" to directory "common/spl".

Signed-off-by: Ying Zhang <b40530@freescale.com>