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/lager_defconfig b/configs/lager_defconfig
index 5d2e99d..3bb4d64 100644
--- a/configs/lager_defconfig
+++ b/configs/lager_defconfig
@@ -14,6 +14,7 @@
 CONFIG_CMD_SPI=y
 CONFIG_CMD_I2C=y
 CONFIG_CMD_USB=y
+CONFIG_CMD_SDRAM=y
 CONFIG_CMD_DHCP=y
 CONFIG_CMD_MII=y
 CONFIG_CMD_PING=y