Convert CONFIG_SYS_MMC_MAX_DEVICE to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_MMC_MAX_DEVICE

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
diff --git a/api/Kconfig b/api/Kconfig
index 382aa4a..eb8d5d0 100644
--- a/api/Kconfig
+++ b/api/Kconfig
@@ -5,4 +5,9 @@
 	help
 	  This option enables the U-Boot API. See api/README for more information.
 
+config SYS_MMC_MAX_DEVICE
+	int  "Maximum number of MMC devices exposed via the API"
+	depends on API
+	default 1
+
 endmenu