mmc: add generic mmc spi driver

This patch supports mmc/sd card with spi interface. It is based on
the generic mmc framework. It works with SDHC and supports multi
blocks read/write.

The crc checksum on data packet is enabled with the def,

There is a subcomamnd "mmc_spi" to setup spi bus and cs at run time.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Signed-off-by: Andy Fleming <afleming@freescale.com>
diff --git a/common/Makefile b/common/Makefile
index 4555716..6f41ce4 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -118,6 +118,7 @@
 COBJS-$(CONFIG_CMD_MII) += cmd_mii.o
 COBJS-$(CONFIG_CMD_MISC) += cmd_misc.o
 COBJS-$(CONFIG_CMD_MMC) += cmd_mmc.o
+COBJS-$(CONFIG_CMD_MMC_SPI) += cmd_mmc_spi.o
 COBJS-$(CONFIG_MP) += cmd_mp.o
 COBJS-$(CONFIG_CMD_MTDPARTS) += cmd_mtdparts.o
 COBJS-$(CONFIG_CMD_NAND) += cmd_nand.o