dm: sandbox: Only enable the sandbox MMC driver when valid

This driver will require generic MMC and block-device support in a future
commit. To avoid test errors, make this change now.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/mmc/Makefile b/drivers/mmc/Makefile
index 6241649..38a172b 100644
--- a/drivers/mmc/Makefile
+++ b/drivers/mmc/Makefile
@@ -38,7 +38,11 @@
 obj-$(CONFIG_SUPPORT_EMMC_RPMB) += rpmb.o
 obj-$(CONFIG_S3C_SDI) += s3c_sdi.o
 obj-$(CONFIG_S5P_SDHCI) += s5p_sdhci.o
+ifdef CONFIG_BLK
+ifdef CONFIG_GENERIC_MMC
 obj-$(CONFIG_SANDBOX) += sandbox_mmc.o
+endif
+endif
 obj-$(CONFIG_SDHCI) += sdhci.o
 obj-$(CONFIG_SH_MMCIF) += sh_mmcif.o
 obj-$(CONFIG_SH_SDHI) += sh_sdhi.o