mmc: add basic SDIO I/O operations

Add command wrappers that simplify register access from SDIO
function drivers.

Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>
diff --git a/drivers/mmc/core/Makefile b/drivers/mmc/core/Makefile
index 71ab3d1..bf7a002 100644
--- a/drivers/mmc/core/Makefile
+++ b/drivers/mmc/core/Makefile
@@ -9,5 +9,6 @@
 obj-$(CONFIG_MMC)		+= mmc_core.o
 mmc_core-y			:= core.o sysfs.o bus.o host.o \
 				   mmc.o mmc_ops.o sd.o sd_ops.o \
-				   sdio.o sdio_ops.o sdio_bus.o
+				   sdio.o sdio_ops.o sdio_bus.o \
+				   sdio_io.o