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/shmin_defconfig b/configs/shmin_defconfig
index 249582f..8570bbc 100644
--- a/configs/shmin_defconfig
+++ b/configs/shmin_defconfig
@@ -14,6 +14,7 @@
 # CONFIG_CMD_LOADB is not set
 # CONFIG_CMD_LOADS is not set
 # CONFIG_CMD_FPGA is not set
+CONFIG_CMD_SDRAM=y
 # CONFIG_CMD_ECHO is not set
 # CONFIG_CMD_ITEST is not set
 # CONFIG_CMD_SOURCE is not set