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>

cmd/Makefile

@@ -110,6 +110,7 @@ obj-$(CONFIG_CMD_LOG) += log.o
 obj-$(CONFIG_CMD_LSBLK) += lsblk.o
 obj-$(CONFIG_CMD_MD5SUM) += md5sum.o
 obj-$(CONFIG_CMD_MEMORY) += mem.o
+obj-$(CONFIG_CMD_MEMINFO) += meminfo.o
 obj-$(CONFIG_CMD_IO) += io.o
 obj-$(CONFIG_CMD_MII) += mii.o
 obj-$(CONFIG_CMD_MISC) += misc.o