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/cmd/Kconfig b/cmd/Kconfig
index cf89fd7..507e8a3 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -785,6 +785,13 @@
 	  Provides a way to save a binary file using the Motorola S-Record
 	  format over the serial line.
 
+config CMD_SDRAM
+	bool "sdram - Print SDRAM configuration information"
+	help
+	  Provides information about attached SDRAM. This assumed that the
+	  SDRAM has an EEPROM with information that can be read using the
+	  I2C bus. This is only available on some boards.
+
 config CMD_SF
 	bool "sf"
 	help