mmc: sandbox: rename CONFIG, fix dependency, and use it in Makefile

[1] Rename CONFIG_SANDBOX_MMC to CONFIG_MMC_SANDBOX for consistency
    I want all MMC driver options prefixed with CONFIG_MMC_.

[2] Fix dependency
    Add necessary depends on to avoid compile error.
    Instead "depends on MMC" is unneeded because this config entry
    resides inside of "if MMC".

[3] Currently, this config symbol is not referenced at all.
    Use it to enable/disable the driver in Makefile.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/drivers/mmc/Kconfig b/drivers/mmc/Kconfig
index df4913b..97d245e 100644
--- a/drivers/mmc/Kconfig
+++ b/drivers/mmc/Kconfig
@@ -187,9 +187,10 @@
 	help
 	  This selects support for the SD/MMC Host Controller on UniPhier SoCs.
 
-config SANDBOX_MMC
+config MMC_SANDBOX
 	bool "Sandbox MMC support"
-	depends on MMC && SANDBOX
+	depends on SANDBOX
+	depends on BLK && DM_MMC_OPS && OF_CONTROL
 	help
 	  This select a dummy sandbox MMC driver. At present this does nothing
 	  other than allow sandbox to be build with MMC support. This