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/ms7720se_defconfig b/configs/ms7720se_defconfig
index a9e3f45..2e5ed21 100644
--- a/configs/ms7720se_defconfig
+++ b/configs/ms7720se_defconfig
@@ -16,6 +16,7 @@
 # CONFIG_CMD_LOADS is not set
 # CONFIG_CMD_FPGA is not set
 CONFIG_CMD_PCMCIA=y
+CONFIG_CMD_SDRAM=y
 # CONFIG_CMD_ECHO is not set
 # CONFIG_CMD_ITEST is not set
 # CONFIG_CMD_SOURCE is not set