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/README b/README
index 9c26168..a9a92d7 100644
--- a/README
+++ b/README
@@ -849,6 +849,7 @@
 		CONFIG_CMD_LOADS	  loads
 		CONFIG_CMD_MD5SUM	  print md5 message digest
 					  (requires CONFIG_CMD_MEMORY and CONFIG_MD5)
+		CONFIG_CMD_MEMINFO	* Display detailed memory information
 		CONFIG_CMD_MEMORY	  md, mm, nm, mw, cp, cmp, crc, base,
 					  loop, loopw, mtest
 		CONFIG_CMD_MISC		  Misc functions like sleep etc