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>