mmc: mmc_spi: Document the 3 local functions

mmc_spi_sendcmd(), mmc_spi_readdata() and mmc_spi_writedata() are
currently undocumented. Add comment blocks to explain the arguments
and the return value.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>