cmd: Move meminfo command into its own file

In preparation for expanding this command, move it into a separate file.
Rename the function to remove the extra underscore. Update the number of
arguments to 1, since 3 is incorrect.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
4 files changed