Convert CONFIG_CMD_SDRAM to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_SDRAM

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
diff --git a/configs/r8a7795_salvator-x_defconfig b/configs/r8a7795_salvator-x_defconfig
index 82b99fe..ade83c2 100644
--- a/configs/r8a7795_salvator-x_defconfig
+++ b/configs/r8a7795_salvator-x_defconfig
@@ -12,6 +12,7 @@
 # CONFIG_CMD_IMLS is not set
 CONFIG_CMD_MMC=y
 CONFIG_CMD_USB=y
+CONFIG_CMD_SDRAM=y
 CONFIG_CMD_DHCP=y
 CONFIG_CMD_MII=y
 CONFIG_CMD_PING=y