dm: sandbox: Convert to use CONFIG_CMD_MMC_OPS

Update the sandbox MMC emulation to use driver model for MMC operations.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/Kconfig b/arch/Kconfig
index 566f044..c43787c 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -63,6 +63,7 @@
 	select DM_I2C
 	select DM_SPI
 	select DM_GPIO
+	select DM_MMC
 
 config SH
 	bool "SuperH architecture"