Add option to display customised memory information

Some boards want to report more than just memory size. For example, it
might be useful to display the memory type (DDR2, DDR3) or manufacturer.

Add a weak function to support this requirement, accessed through a new
'meminfo' command.

Any example of the DRAM: output is below, just for illustration:

SMDK5250 # meminfo
DRAM:  2 GiB Elpida DDR3 @ 800MHz

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/config_cmd_all.h b/include/config_cmd_all.h
index e82f642..2a82e19 100644
--- a/include/config_cmd_all.h
+++ b/include/config_cmd_all.h
@@ -56,6 +56,7 @@
 #define CONFIG_CMD_LICENSE	/* console license display	*/
 #define CONFIG_CMD_LOADB	/* loadb			*/
 #define CONFIG_CMD_LOADS	/* loads			*/
+#define CONFIG_CMD_MEMINFO	/* meminfo			*/
 #define CONFIG_CMD_MEMORY	/* md mm nm mw cp cmp crc base loop mtest */
 #define CONFIG_CMD_MFSL		/* FSL support for Microblaze	*/
 #define CONFIG_CMD_MII		/* MII support			*/